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