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/dependencies.go

@@ -23,6 +23,7 @@ func checkDependencies(p *model.Pipeline) []Finding {
 			if !exists {
 				findings = append(findings, Finding{
 					Severity: Error,
+					Rule:     RuleUnknownDependency,
 					Job:      name,
 					File:     job.File,
 					Line:     job.Line,
@@ -35,6 +36,7 @@ func checkDependencies(p *model.Pipeline) []Finding {
 				if depHasStage && depIdx >= jobStageIdx {
 					findings = append(findings, Finding{
 						Severity: Error,
+						Rule:     RuleDependencyStage,
 						Job:      name,
 						File:     job.File,
 						Line:     job.Line,