feat(linter): rules:needs: validation (GL044)

Add GL044 to validate jobs listed in rules:needs: overrides (GitLab CI
16.4+). rules:needs: lets a specific rule override the job's top-level
needs: list; any referenced job must exist in the pipeline. Unknown jobs
produce an error; optional: true entries produce a warning, matching the
GL027 behaviour for top-level needs:. Cross-pipeline needs and skipped
jobs (when a context is active) are excluded from checking.

Implementation:
- model.Rule gains a Needs []any field (yaml:"needs")
- checkRulesNeeds(p, skipped) added to needs.go; wired into Lint
- GL044 / RuleRulesNeedsUnknown added to rules.go and explain.go
- 6 unit tests + 2 testdata fixtures; task validate updated

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

internal/linter/needs_test.go

@@ -31,6 +31,129 @@ func TestCheckNeeds_SkippedJob(t *testing.T) {
 	}
 }
 
+// TestCheckRulesNeeds_UnknownJob verifies that an unknown job in rules:needs: produces GL044.
+func TestCheckRulesNeeds_UnknownJob(t *testing.T) {
+	p := &model.Pipeline{
+		Stages: []string{"build", "test"},
+		Jobs: map[string]model.Job{
+			"build-job": {
+				Name:   "build-job",
+				Stage:  "build",
+				Script: []any{"make"},
+			},
+			"test-job": {
+				Name:   "test-job",
+				Stage:  "test",
+				Script: []any{"make test"},
+				Rules: []model.Rule{
+					{
+						If:    `$CI_COMMIT_BRANCH == "main"`,
+						Needs: []any{"build-job", "nonexistent"},
+					},
+				},
+			},
+		},
+	}
+	findings := checkRulesNeeds(p, nil)
+	var got bool
+	for _, f := range findings {
+		if f.Rule == RuleRulesNeedsUnknown && f.Severity == Error {
+			got = true
+		}
+	}
+	if !got {
+		t.Errorf("expected GL044 error for unknown rules:needs: job; got %v", findings)
+	}
+}
+
+// TestCheckRulesNeeds_KnownJob verifies no finding when all rules:needs: jobs exist.
+func TestCheckRulesNeeds_KnownJob(t *testing.T) {
+	p := &model.Pipeline{
+		Stages: []string{"build", "test"},
+		Jobs: map[string]model.Job{
+			"build-job": {Name: "build-job", Stage: "build", Script: []any{"make"}},
+			"test-job": {
+				Name:   "test-job",
+				Stage:  "test",
+				Script: []any{"make test"},
+				Rules:  []model.Rule{{Needs: []any{"build-job"}}},
+			},
+		},
+	}
+	if findings := checkRulesNeeds(p, nil); len(findings) != 0 {
+		t.Errorf("expected no findings for valid rules:needs:; got %v", findings)
+	}
+}
+
+// TestCheckRulesNeeds_Optional verifies that optional: true downgrades to Warning.
+func TestCheckRulesNeeds_Optional(t *testing.T) {
+	p := &model.Pipeline{
+		Jobs: map[string]model.Job{
+			"test-job": {
+				Name:   "test-job",
+				Script: []any{"make test"},
+				Rules: []model.Rule{
+					{Needs: []any{map[string]any{"job": "ghost", "optional": true}}},
+				},
+			},
+		},
+	}
+	findings := checkRulesNeeds(p, nil)
+	if len(findings) != 1 || findings[0].Severity != Warning {
+		t.Errorf("optional unknown rules:needs: should produce Warning; got %v", findings)
+	}
+}
+
+// TestCheckRulesNeeds_CrossPipeline verifies that cross-pipeline needs are ignored.
+func TestCheckRulesNeeds_CrossPipeline(t *testing.T) {
+	p := &model.Pipeline{
+		Jobs: map[string]model.Job{
+			"test-job": {
+				Name:   "test-job",
+				Script: []any{"make test"},
+				Rules: []model.Rule{
+					{Needs: []any{map[string]any{"pipeline": "other", "job": "j"}}},
+				},
+			},
+		},
+	}
+	if findings := checkRulesNeeds(p, nil); len(findings) != 0 {
+		t.Errorf("cross-pipeline rules:needs: should be ignored; got %v", findings)
+	}
+}
+
+// TestCheckRulesNeeds_SkippedJob verifies that a skipped job's rules:needs: violations are suppressed.
+func TestCheckRulesNeeds_SkippedJob(t *testing.T) {
+	p := &model.Pipeline{
+		Jobs: map[string]model.Job{
+			"test-job": {
+				Name:   "test-job",
+				Script: []any{"make test"},
+				Rules:  []model.Rule{{Needs: []any{"nonexistent"}}},
+			},
+		},
+	}
+	if findings := checkRulesNeeds(p, map[string]bool{"test-job": true}); len(findings) != 0 {
+		t.Errorf("skipped job: expected no findings; got %v", findings)
+	}
+}
+
+// TestCheckRulesNeeds_NoNeeds verifies no findings when rules have no needs: override.
+func TestCheckRulesNeeds_NoNeeds(t *testing.T) {
+	p := &model.Pipeline{
+		Jobs: map[string]model.Job{
+			"test-job": {
+				Name:   "test-job",
+				Script: []any{"make test"},
+				Rules:  []model.Rule{{When: "on_success"}},
+			},
+		},
+	}
+	if findings := checkRulesNeeds(p, nil); len(findings) != 0 {
+		t.Errorf("rules without needs: should produce no findings; got %v", findings)
+	}
+}
+
 // TestCheckNeeds_StageOrder verifies that a job needing a job in a later stage
 // produces RuleNeedsStageOrder (line 64-76 in needs.go).
 func TestCheckNeeds_StageOrder(t *testing.T) {