feat(cli): inject GitLab predefined variables into the simulation context

Two categories of predefined variables are now injected automatically:

1. Always-available (CI=true, GITLAB_CI=true): set at lowest priority for
   every non-empty context so that rules:if: expressions like '$CI == "true"'
   evaluate correctly without requiring --var.

2. MR-specific (CI_MERGE_REQUEST_IID, CI_MERGE_REQUEST_SOURCE_BRANCH_NAME,
   CI_MERGE_REQUEST_TARGET_BRANCH_NAME, …): injected as placeholder values
   when --source merge_request_event is given, so MR-gated jobs evaluate
   as active rather than silently skipped. CI_MERGE_REQUEST_SOURCE_BRANCH_NAME
   is derived from --branch when provided.

All injected defaults are non-pinned: --var overrides them, and pipeline
variables: blocks can also override via Inject(). The empty context (no
flags at all) is unchanged — no predefined vars are injected.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

cmd/glint/main.go

@@ -258,7 +258,10 @@ Note: when none of --branch, --tag, --source, or --var are given, glint
 detects the current git branch automatically and uses it as the default
 (falling back to 'main' when not inside a git repository or in detached-HEAD
 state). --source defaults to 'push' so that rules:if: expressions are always
-evaluated.
+evaluated. GitLab's always-available predefined variables (CI=true,
+GITLAB_CI=true) are injected automatically; MR-specific variables
+(CI_MERGE_REQUEST_IID, CI_MERGE_REQUEST_SOURCE_BRANCH_NAME, …) are injected
+when --source merge_request_event is given. Override any with --var.
 
 Examples:
   glint check .gitlab-ci.yml
@@ -269,6 +272,8 @@ Examples:
   glint check --branch develop .gitlab-ci.yml
   glint check --tag v1.0.0 .gitlab-ci.yml
   glint check --source merge_request_event .gitlab-ci.yml
+  glint check --source merge_request_event --branch feature/my-branch .gitlab-ci.yml
+  glint check --source merge_request_event --var CI_MERGE_REQUEST_IID=42 .gitlab-ci.yml
   glint check --list-vars .gitlab-ci.yml
   GITLAB_TOKEN=glpat-xxxx glint check .gitlab-ci.yml
   glint check --token glpat-xxxx --gitlab-url https://gitlab.example.com .gitlab-ci.yml
@@ -619,7 +624,10 @@ Note: when none of --branch, --tag, --source, or --var are given, glint
 detects the current git branch automatically and uses it as the default
 (falling back to 'main' when not inside a git repository or in detached-HEAD
 state). --source defaults to 'push' so that rules:if: expressions are always
-evaluated.
+evaluated. GitLab's always-available predefined variables (CI=true,
+GITLAB_CI=true) are injected automatically; MR-specific variables
+(CI_MERGE_REQUEST_IID, CI_MERGE_REQUEST_SOURCE_BRANCH_NAME, …) are injected
+when --source merge_request_event is given. Override any with --var.
 
 Examples:
   glint graph .gitlab-ci.yml

internal/cicontext/context.go

@@ -14,6 +14,32 @@ type Context struct {
 	changedFiles []string        // nil = not provided (permissive); non-nil = known set of changed files
 }
 
+// gitlabAlwaysVars are GitLab predefined variables that are constant across
+// every pipeline, instance, and project. Injected at the lowest priority so
+// that pipeline variables: blocks and --var overrides can still win.
+var gitlabAlwaysVars = map[string]string{
+	"CI":        "true",
+	"GITLAB_CI": "true",
+}
+
+// gitlabMRVars are predefined variables GitLab injects only for
+// CI_PIPELINE_SOURCE=merge_request_event pipelines. Placeholder values are
+// used so that rules:if: expressions that gate on MR context evaluate
+// correctly. All can be overridden with --var.
+//
+// CI_MERGE_REQUEST_SOURCE_BRANCH_NAME is left empty here and filled from
+// CI_COMMIT_BRANCH (the --branch flag) when available.
+var gitlabMRVars = map[string]string{
+	"CI_MERGE_REQUEST_ID":                  "1",
+	"CI_MERGE_REQUEST_IID":                 "1",
+	"CI_MERGE_REQUEST_SOURCE_BRANCH_NAME":  "",
+	"CI_MERGE_REQUEST_TARGET_BRANCH_NAME":  "main",
+	"CI_MERGE_REQUEST_PROJECT_PATH":        "namespace/project",
+	"CI_MERGE_REQUEST_TITLE":               "Draft: placeholder",
+	"CI_MERGE_REQUEST_LABELS":              "",
+	"CI_OPEN_MERGE_REQUESTS":               "namespace/project!1",
+}
+
 // New builds a Context from high-level shortcut values and optional KEY=VALUE
 // overrides. Predefined CI variables are derived from the shortcuts so callers
 // do not need to know their exact names.
@@ -21,15 +47,18 @@ type Context struct {
 // Returns an empty Context (IsEmpty() == true) when all inputs are zero values,
 // preserving the existing linting behaviour when no context flags are given.
 //
-// Override priority (highest wins): extraVars > branch/tag/source shortcuts.
+// Override priority (highest wins): extraVars > branch/tag/source shortcuts >
-// Both shortcut-derived and extraVar variables are pinned — they will not be
+// pipeline variables (via Inject) > GitLab predefined defaults.
-// overwritten by Inject (used for pipeline-level and workflow-rule variables).
 func New(branch, tag, source string, extraVars []string) *Context {
 	if branch == "" && tag == "" && source == "" && len(extraVars) == 0 {
 		return &Context{}
 	}
 
-	vars := make(map[string]string)
+	// Seed with always-available GitLab predefined variables at lowest priority.
+	vars := make(map[string]string, len(gitlabAlwaysVars)+8)
+	for k, v := range gitlabAlwaysVars {
+		vars[k] = v
+	}
 	pinned := make(map[string]bool)
 
 	pin := func(k, v string) {
@@ -62,6 +91,22 @@ func New(branch, tag, source string, extraVars []string) *Context {
 		vars["CI_DEFAULT_BRANCH"] = "main"
 	}
 
+	// For MR pipelines, inject placeholder values for the predefined MR
+	// variables so that rules:if: expressions like '$CI_MERGE_REQUEST_IID'
+	// evaluate as non-empty (truthy). These are non-pinned so --var overrides
+	// them and pipeline variables: can also override via Inject.
+	if source == "merge_request_event" {
+		for k, v := range gitlabMRVars {
+			if !pinned[k] {
+				vars[k] = v
+			}
+		}
+		// Derive source branch from --branch when available.
+		if branch != "" && vars["CI_MERGE_REQUEST_SOURCE_BRANCH_NAME"] == "" {
+			vars["CI_MERGE_REQUEST_SOURCE_BRANCH_NAME"] = branch
+		}
+	}
+
 	// KEY=VALUE overrides win over shortcuts and everything else.
 	for _, kv := range extraVars {
 		k, v, ok := strings.Cut(kv, "=")

internal/cicontext/context_test.go

@@ -78,6 +78,66 @@ func TestNew_DefaultBranch(t *testing.T) {
 	}
 }
 
+func TestNew_PredefinedAlwaysVars(t *testing.T) {
+	ctx := New("main", "", "", nil)
+	if ctx.Get("CI") != "true" {
+		t.Errorf("CI should be 'true', got %q", ctx.Get("CI"))
+	}
+	if ctx.Get("GITLAB_CI") != "true" {
+		t.Errorf("GITLAB_CI should be 'true', got %q", ctx.Get("GITLAB_CI"))
+	}
+}
+
+func TestNew_PredefinedAlwaysVars_EmptyContext(t *testing.T) {
+	// Always-vars must NOT be injected when the context is empty (no flags).
+	ctx := New("", "", "", nil)
+	if ctx.Get("CI") != "" {
+		t.Errorf("CI should be empty in empty context, got %q", ctx.Get("CI"))
+	}
+}
+
+func TestNew_PredefinedAlwaysVars_OverridableByVar(t *testing.T) {
+	ctx := New("main", "", "", []string{"CI=false"})
+	if ctx.Get("CI") != "false" {
+		t.Errorf("--var should override CI, got %q", ctx.Get("CI"))
+	}
+}
+
+func TestNew_PredefinedAlwaysVars_OverridableByInject(t *testing.T) {
+	ctx := New("main", "", "", nil)
+	ctx.Inject("CI", "custom")
+	if ctx.Get("CI") != "custom" {
+		t.Errorf("Inject should override non-pinned CI, got %q", ctx.Get("CI"))
+	}
+}
+
+func TestNew_MRVars(t *testing.T) {
+	ctx := New("feature/my-branch", "", "merge_request_event", nil)
+	if ctx.Get("CI_MERGE_REQUEST_IID") != "1" {
+		t.Errorf("CI_MERGE_REQUEST_IID should be '1', got %q", ctx.Get("CI_MERGE_REQUEST_IID"))
+	}
+	if ctx.Get("CI_MERGE_REQUEST_SOURCE_BRANCH_NAME") != "feature/my-branch" {
+		t.Errorf("source branch should match --branch, got %q", ctx.Get("CI_MERGE_REQUEST_SOURCE_BRANCH_NAME"))
+	}
+	if ctx.Get("CI_MERGE_REQUEST_TARGET_BRANCH_NAME") != "main" {
+		t.Errorf("target branch should be 'main', got %q", ctx.Get("CI_MERGE_REQUEST_TARGET_BRANCH_NAME"))
+	}
+}
+
+func TestNew_MRVars_NotInjectedForNonMR(t *testing.T) {
+	ctx := New("main", "", "push", nil)
+	if ctx.Get("CI_MERGE_REQUEST_IID") != "" {
+		t.Errorf("CI_MERGE_REQUEST_IID should be empty for push pipeline, got %q", ctx.Get("CI_MERGE_REQUEST_IID"))
+	}
+}
+
+func TestNew_MRVars_OverridableByVar(t *testing.T) {
+	ctx := New("", "", "merge_request_event", []string{"CI_MERGE_REQUEST_IID=42"})
+	if ctx.Get("CI_MERGE_REQUEST_IID") != "42" {
+		t.Errorf("--var should override CI_MERGE_REQUEST_IID, got %q", ctx.Get("CI_MERGE_REQUEST_IID"))
+	}
+}
+
 func TestInject(t *testing.T) {
 	ctx := New("main", "", "", nil)
 	// pinned var should not be overwritten