feat(linter): context-scoped needs:/dependencies: cross-checks

When glint check runs in single-context mode (--branch, --tag, --source,
or --var), jobs that resolve to JobSkipped against that context are now
excluded from needs: and dependencies: cross-job checks (GL027-GL031).
This eliminates false-positive errors for jobs intentionally gated to
specific pipeline events (e.g. a deploy job with rules:if: CI_COMMIT_TAG
no longer triggers GL027 on branch pipelines).

linter.Lint now accepts a skipped map[string]bool (nil = check all jobs);
checkNeeds and checkDependencies skip jobs present in the map. Multi-context
mode (--context) passes nil, so all jobs are checked regardless of context.

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

internal/linter/needs.go

@@ -12,7 +12,7 @@ type needEntry struct {
 	optional bool // true when the needs entry carries optional: true
 }
 
-func checkNeeds(p *model.Pipeline) []Finding {
+func checkNeeds(p *model.Pipeline, skipped map[string]bool) []Finding {
 	var findings []Finding
 
 	// Build a stage-index map for ordering checks.
@@ -26,6 +26,9 @@ func checkNeeds(p *model.Pipeline) []Finding {
 	needsGraph := make(map[string][]string)
 
 	for name, job := range p.Jobs {
+		if skipped[name] {
+			continue
+		}
 		if len(job.Needs) == 0 {
 			continue
 		}