feat(linter): propagate workflow:rules:variables: into job rule evaluation

workflow:rules: can define variables: on matching rules (GitLab CI 15.0+).
These variables are now injected into the evaluation context before job
rules:if: expressions are evaluated, making patterns like:

  workflow:
    rules:
      - if: '$CI_COMMIT_BRANCH == "main"'
        variables:
          DEPLOY_TARGET: production
  deploy:
    rules:
      - if: '$DEPLOY_TARGET == "production"'

work correctly with glint check --branch main.

Changes:
- model.Rule: add Variables map[string]any field (yaml:"variables")
- cicontext.Context: add pinned map tracking which vars must not be
  overwritten; New() pins all shortcut and --var variables; add
  Inject(key, value) which writes only when key is not pinned
- cicontext.ExtractStringVars: shared helper that converts map[string]any
  variable blocks (plain string or {value:...} form) to map[string]string
- cicontext.EvalWorkflow: returns (bool, map[string]string) — the vars of
  the matching workflow rule alongside the runs/no-runs result
- cmd/glint/main.go: enrichContext() injects pipeline-level variable
  defaults then workflow-rule variables before printContext; applied in
  both cmdCheck and cmdGraph

Injection priority (highest wins):
  --var CLI overrides > --branch/--tag/--source shortcuts
  > workflow-rule variables > pipeline variables: defaults

Adds 15 unit tests (TestEvalWorkflow, TestContextInject,
TestExtractStringVars, TestWorkflowVarsJobEval) and a testdata fixture
(workflow_vars.yml) validated across four branch contexts.

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

cmd/glint/main.go

@@ -147,6 +147,7 @@ Examples:
 
 	ctx := cicontext.New(*branch, *tag, *source, vars)
 	if !ctx.IsEmpty() {
+		enrichContext(ctx, p)
 		printContext(p, ctx)
 	}
 
@@ -272,6 +273,9 @@ Examples:
 	resolver.Resolve(p)                        //nolint:errcheck
 
 	ctx := cicontext.New(*branch, *tag, *source, vars)
+	if !ctx.IsEmpty() {
+		enrichContext(ctx, p)
+	}
 
 	switch mode {
 	case "default":
@@ -300,6 +304,24 @@ Examples:
 	}
 }
 
+// enrichContext injects pipeline-level variable defaults and then
+// workflow-rule-generated variables into ctx before job evaluation.
+// Injection respects pinned variables (--branch/--tag/--source/--var always win).
+func enrichContext(ctx *cicontext.Context, p *model.Pipeline) {
+	// Pipeline variables: injected as defaults (lowest priority).
+	for k, v := range cicontext.ExtractStringVars(p.Variables) {
+		ctx.Inject(k, v)
+	}
+	// Workflow rules: evaluate to find which rule matches, then inject its variables.
+	runs, ruleVars := cicontext.EvalWorkflow(p, ctx)
+	if !runs {
+		fmt.Fprintln(os.Stderr, "[WARNING] workflow:rules: pipeline would not start for this context")
+	}
+	for k, v := range ruleVars {
+		ctx.Inject(k, v)
+	}
+}
+
 func printContext(p *model.Pipeline, ctx *cicontext.Context) {
 	fmt.Printf("Context: %s\n\n", ctx.Summary())