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/dependencies.go

@@ -6,7 +6,7 @@ import (
 	"git.k3nny.fr/glint/internal/model"
 )
 
-func checkDependencies(p *model.Pipeline) []Finding {
+func checkDependencies(p *model.Pipeline, skipped map[string]bool) []Finding {
 	stageIndex := make(map[string]int, len(p.Stages))
 	for i, s := range p.Stages {
 		stageIndex[s] = i
@@ -14,6 +14,9 @@ func checkDependencies(p *model.Pipeline) []Finding {
 
 	var findings []Finding
 	for name, job := range p.Jobs {
+		if skipped[name] {
+			continue
+		}
 		if len(job.Dependencies) == 0 {
 			continue
 		}