feat(linter): GL033 static dead-rules detection

Add rule GL033 that warns when every rule in a job's rules: block has
an explicit when: never, making the job permanently excluded from any
pipeline run. This is a pure static check — no if: evaluation or context
required. Only rules with literal when: never trigger it; rules with no
when: (defaults to on_success), when: manual, when: always, or
when: on_failure are treated as reachable.

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

internal/linter/dead_rules_test.go

@@ -0,0 +1,90 @@
+package linter
+
+import (
+	"testing"
+
+	"git.k3nny.fr/glint/internal/model"
+)
+
+func TestCheckDeadRules(t *testing.T) {
+	cases := []struct {
+		name     string
+		rules    []model.Rule
+		wantHit  bool // whether GL033 should fire
+	}{
+		{
+			name:    "no rules — not dead",
+			rules:   nil,
+			wantHit: false,
+		},
+		{
+			name:    "single bare when:never — dead",
+			rules:   []model.Rule{{When: "never"}},
+			wantHit: true,
+		},
+		{
+			name: "all rules when:never with if — dead",
+			rules: []model.Rule{
+				{If: `$CI_COMMIT_BRANCH == "main"`, When: "never"},
+				{If: `$CI_COMMIT_BRANCH == "develop"`, When: "never"},
+				{When: "never"},
+			},
+			wantHit: true,
+		},
+		{
+			name: "first rule on_success — not dead",
+			rules: []model.Rule{
+				{If: `$CI_COMMIT_BRANCH == "main"`, When: "on_success"},
+				{When: "never"},
+			},
+			wantHit: false,
+		},
+		{
+			name: "rule with empty when (defaults to on_success) — not dead",
+			rules: []model.Rule{
+				{If: `$CI_COMMIT_BRANCH == "main"`},
+				{When: "never"},
+			},
+			wantHit: false,
+		},
+		{
+			name:    "when:manual — not dead",
+			rules:   []model.Rule{{When: "manual"}},
+			wantHit: false,
+		},
+		{
+			name:    "when:always — not dead",
+			rules:   []model.Rule{{When: "always"}},
+			wantHit: false,
+		},
+		{
+			name:    "when:on_failure — not dead",
+			rules:   []model.Rule{{When: "on_failure"}},
+			wantHit: false,
+		},
+		{
+			name: "mixed never and manual — not dead",
+			rules: []model.Rule{
+				{If: `$CI_COMMIT_BRANCH == "main"`, When: "never"},
+				{When: "manual"},
+			},
+			wantHit: false,
+		},
+	}
+
+	for _, tc := range cases {
+		t.Run(tc.name, func(t *testing.T) {
+			job := model.Job{Rules: tc.rules}
+			findings := checkDeadRules("test-job", job)
+			hit := false
+			for _, f := range findings {
+				if f.Rule == RuleDeadRules {
+					hit = true
+				}
+			}
+			if hit != tc.wantHit {
+				t.Errorf("checkDeadRules: got hit=%v, want hit=%v; findings=%v", hit, tc.wantHit, findings)
+			}
+		})
+	}
+}