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>

testdata/workflow_vars.yml

@@ -0,0 +1,51 @@
+---
+# workflow_vars.yml
+# Exercises workflow:rules:variables: injection.
+# The matching workflow rule sets DEPLOY_TARGET; job rules use it.
+#
+# Expected behaviour per context:
+#   (no context)       → all jobs active (no context evaluation)
+#   --branch main      → deploy-prod active, deploy-staging skipped
+#   --branch develop   → deploy-prod skipped, deploy-staging active
+#   --branch feat/x    → deploy-prod skipped, deploy-staging skipped (manual)
+
+stages:
+  - build
+  - deploy
+
+variables:
+  DEPLOY_TARGET:
+    value: ""
+    description: "Deployment target — set by workflow rules"
+
+workflow:
+  rules:
+    - if: '$CI_COMMIT_BRANCH == "main"'
+      variables:
+        DEPLOY_TARGET: production
+    - if: '$CI_COMMIT_BRANCH == "develop"'
+      variables:
+        DEPLOY_TARGET: staging
+    - when: always
+
+build:
+  stage: build
+  script: make build
+  rules:
+    - when: always
+
+deploy-prod:
+  stage: deploy
+  script: make deploy ENV=production
+  rules:
+    - if: '$DEPLOY_TARGET == "production"'
+      when: on_success
+    - when: never
+
+deploy-staging:
+  stage: deploy
+  script: make deploy ENV=staging
+  rules:
+    - if: '$DEPLOY_TARGET == "staging"'
+      when: on_success
+    - when: never