feat(linter): add structured rule IDs (GL001–GL031)

Every Finding now carries a stable Rule string field with a GL### code.
The ID appears in output between the source location and the message:

  [ERROR] job "deploy" (ci.yml:14) GL003: missing required field 'script'
  [WARNING] (ci.yml) GL001: no stages defined

Rules:
  GL001 no-stages          GL002 workflow-when       GL003 missing-script
  GL004 unknown-stage      GL005 only-rules-conflict GL006 except-rules-conflict
  GL007 deprecated-only    GL008 invalid-when        GL009 delayed-no-start-in
  GL010 start-in-no-delayed GL011 invalid-parallel   GL012 invalid-retry
  GL013 invalid-retry-when GL014 invalid-allow-failure GL015 invalid-interruptible
  GL016 trigger-with-script GL017 invalid-trigger    GL018 invalid-coverage
  GL019 invalid-release    GL020 invalid-environment GL021 invalid-artifacts
  GL022 pages-public       GL023 invalid-cache       GL024 invalid-rules-when
  GL025 invalid-image      GL026 invalid-inherit     GL027 needs-unknown
  GL028 needs-stage-order  GL029 needs-cycle         GL030 unknown-dependency
  GL031 dependency-stage

Changes:
- internal/linter/rules.go: new file with all 31 constants + doc comments
- linter.Finding: add Rule string field; String() inserts it before the
  message colon when non-empty; format unchanged when Rule == ""
- All Finding{} literals in linter.go, keywords.go, needs.go,
  dependencies.go updated with the correct Rule: constant
- README.md lint rules table: new ID column added to all four sections
- CHANGELOG.md: entry in [Unreleased]

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

internal/linter/rules.go

@@ -0,0 +1,110 @@
+package linter
+
+// Rule ID constants. Each ID is stable across versions and uniquely identifies
+// one lint check. Use these when filtering output, writing suppression rules,
+// or referencing a check in documentation.
+//
+// Prefix GL = Glint / GitLab CI lint.
+// Numbering is sequential by category; gaps may appear as rules are added.
+const (
+	// ── Pipeline-level ──────────────────────────────────────────────────────
+
+	// GL001: no stages: block defined; GitLab falls back to default stages.
+	RuleNoStages = "GL001"
+
+	// GL002: workflow.rules[n].when has an invalid value (only always/never allowed).
+	RuleWorkflowWhen = "GL002"
+
+	// ── Job structure ────────────────────────────────────────────────────────
+
+	// GL003: job is missing a required script: (or run:) field.
+	RuleMissingScript = "GL003"
+
+	// GL004: job references a stage not declared in stages:.
+	RuleUnknownStage = "GL004"
+
+	// GL005: only: and rules: used together on the same job.
+	RuleOnlyRulesConflict = "GL005"
+
+	// GL006: except: and rules: used together on the same job.
+	RuleExceptRulesConflict = "GL006"
+
+	// GL007: only:/except: used (deprecated; prefer rules:).
+	RuleDeprecatedOnly = "GL007"
+
+	// ── Keyword constraints ──────────────────────────────────────────────────
+
+	// GL008: when: has an invalid value.
+	RuleInvalidWhen = "GL008"
+
+	// GL009: when: delayed without start_in:.
+	RuleDelayedNoStartIn = "GL009"
+
+	// GL010: start_in: set when when: is not delayed.
+	RuleStartInNoDelayed = "GL010"
+
+	// GL011: parallel: value is invalid (integer out of range or map missing matrix:).
+	RuleInvalidParallel = "GL011"
+
+	// GL012: retry: integer is out of range 0–2, or retry: is neither int nor map.
+	RuleInvalidRetry = "GL012"
+
+	// GL013: retry.when: contains an unrecognised failure type.
+	RuleInvalidRetryWhen = "GL013"
+
+	// GL014: allow_failure: is not a boolean or a map with exit_codes:.
+	RuleInvalidAllowFailure = "GL014"
+
+	// GL015: interruptible: is not a boolean.
+	RuleInvalidInterruptible = "GL015"
+
+	// GL016: trigger: job also defines script: (mutually exclusive).
+	RuleTriggerWithScript = "GL016"
+
+	// GL017: trigger: map does not specify project: or include:.
+	RuleInvalidTrigger = "GL017"
+
+	// GL018: coverage: is not a regex pattern wrapped in /.
+	RuleInvalidCoverage = "GL018"
+
+	// GL019: release: is missing required tag_name:, or is not a map.
+	RuleInvalidRelease = "GL019"
+
+	// GL020: environment: has an invalid url/action configuration.
+	RuleInvalidEnvironment = "GL020"
+
+	// GL021: artifacts: has an invalid when/expose_as configuration.
+	RuleInvalidArtifacts = "GL021"
+
+	// GL022: pages job artifacts.paths does not include public/.
+	RulePagesPublic = "GL022"
+
+	// GL023: cache: has an invalid when/policy value.
+	RuleInvalidCache = "GL023"
+
+	// GL024: rules[n].when has an invalid value.
+	RuleInvalidRulesWhen = "GL024"
+
+	// GL025: image: map form is missing a name: key.
+	RuleInvalidImage = "GL025"
+
+	// GL026: inherit.default or inherit.variables is not a boolean or list.
+	RuleInvalidInherit = "GL026"
+
+	// ── Cross-job graph ──────────────────────────────────────────────────────
+
+	// GL027: needs: references a job that does not exist in the pipeline.
+	RuleNeedsUnknown = "GL027"
+
+	// GL028: needs: references a job in a later stage than the current job.
+	RuleNeedsStageOrder = "GL028"
+
+	// GL029: circular dependency detected in the needs: graph.
+	RuleNeedsCycle = "GL029"
+
+	// GL030: dependencies: references a job that does not exist.
+	RuleUnknownDependency = "GL030"
+
+	// GL031: dependencies: references a job in the same or a later stage.
+	RuleDependencyStage = "GL031"
+)