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/samba_test.go

@@ -35,7 +35,7 @@ func TestSambaCI(t *testing.T) {
 		t.Logf("extends warning: job %q extends unknown %q", w.Job, w.Base)
 	}
 
-	findings := linter.Lint(p)
+	findings := linter.Lint(p, nil)
 
 	for _, f := range findings {
 		if f.Severity == linter.Error {
@@ -78,7 +78,7 @@ func TestSambaCIEntryFiles(t *testing.T) {
 				t.Logf("extends warning: job %q extends unknown %q", w.Job, w.Base)
 			}
 
-			findings := linter.Lint(p)
+			findings := linter.Lint(p, nil)
 			for _, f := range findings {
 				if f.Severity == linter.Error {
 					t.Errorf("unexpected error finding: %s", f)