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>

internal/model/pipeline.go

@@ -80,10 +80,11 @@ type Job struct {
 }
 
 type Rule struct {
-	If      string `yaml:"if"`
-	When    string `yaml:"when"`
-	Changes any    `yaml:"changes"` // []string or {paths,compare_to} map
-	Exists  any    `yaml:"exists"`  // []string or map form
+	If        string         `yaml:"if"`
+	When      string         `yaml:"when"`
+	Changes   any            `yaml:"changes"`   // []string or {paths,compare_to} map
+	Exists    any            `yaml:"exists"`    // []string or map form
+	Variables map[string]any `yaml:"variables"` // set/override variables when rule matches (GitLab CI 15.0+)
 }
 
 // ReservedKeys are top-level GitLab CI keys that are NOT job definitions.