feat(linter): glint explain, GL042 rules:if: reachability, GL043 inherit completeness

- `glint explain <RULE>`: new subcommand printing rule description,
  rationale, bad-YAML example and fix for every GL001–GL043 rule.
  `glint explain` (no arg) lists all rules with ID, severity, title.
  Rule IDs are case-insensitive.

- GL042 (rules:if: evaluated reachability): warns when every rules:if:
  condition evaluates to false given the values of variables declared in
  the pipeline YAML, making the job statically unreachable. Conservative:
  only fires when all referenced variables are declared in YAML; predefined
  CI_* / GITLAB_* variables are skipped to avoid false positives.

- GL043 (inherit: completeness): warns when inherit: default: is declared
  but there is no default: block in the pipeline (dead declaration), or
  when the list form names fields not set in the default: block.

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

internal/linter/if_reachability_test.go

@@ -0,0 +1,152 @@
+package linter
+
+import (
+	"testing"
+
+	"git.k3nny.fr/glint/internal/model"
+)
+
+func TestCheckRulesIfReachability(t *testing.T) {
+	makeJob := func(vars map[string]any, rules []model.Rule) model.Job {
+		return model.Job{Variables: vars, Rules: rules}
+	}
+
+	cases := []struct {
+		name        string
+		pipelineVars map[string]any
+		jobs        map[string]model.Job
+		wantHit     []string // job names that should produce GL042
+		wantMiss    []string // job names that should NOT produce GL042
+	}{
+		{
+			name:         "declared var always false — GL042 fires",
+			pipelineVars: map[string]any{"DEPLOY": "false"},
+			jobs: map[string]model.Job{
+				"deploy": makeJob(nil, []model.Rule{
+					{If: `$DEPLOY == "true"`, When: "on_success"},
+				}),
+			},
+			wantHit:  []string{"deploy"},
+			wantMiss: nil,
+		},
+		{
+			name:         "declared var matches — no finding",
+			pipelineVars: map[string]any{"DEPLOY": "true"},
+			jobs: map[string]model.Job{
+				"deploy": makeJob(nil, []model.Rule{
+					{If: `$DEPLOY == "true"`, When: "on_success"},
+				}),
+			},
+			wantHit:  nil,
+			wantMiss: []string{"deploy"},
+		},
+		{
+			name:         "predefined CI_ var — skip check conservatively",
+			pipelineVars: nil,
+			jobs: map[string]model.Job{
+				"branch-job": makeJob(nil, []model.Rule{
+					{If: `$CI_COMMIT_BRANCH == "never-exists"`, When: "on_success"},
+				}),
+			},
+			wantHit:  nil,
+			wantMiss: []string{"branch-job"},
+		},
+		{
+			name:         "undeclared var — skip check conservatively",
+			pipelineVars: nil,
+			jobs: map[string]model.Job{
+				"my-job": makeJob(nil, []model.Rule{
+					{If: `$UNKNOWN_VAR == "x"`, When: "on_success"},
+				}),
+			},
+			wantHit:  nil,
+			wantMiss: []string{"my-job"},
+		},
+		{
+			name:         "unconditional rule — job can always activate",
+			pipelineVars: map[string]any{"DEPLOY": "false"},
+			jobs: map[string]model.Job{
+				"my-job": makeJob(nil, []model.Rule{
+					{If: `$DEPLOY == "true"`, When: "never"},
+					{When: "on_success"},
+				}),
+			},
+			wantHit:  nil,
+			wantMiss: []string{"my-job"},
+		},
+		{
+			name:         "all rules when:never — GL042 fires (not GL033 territory overlap)",
+			pipelineVars: map[string]any{"DEPLOY": "false"},
+			jobs: map[string]model.Job{
+				"my-job": makeJob(nil, []model.Rule{
+					{If: `$DEPLOY == "true"`, When: "on_success"},
+					{When: "never"},
+				}),
+			},
+			// Rule 1 evaluates to false; Rule 2 is explicit never → all dead.
+			wantHit:  []string{"my-job"},
+			wantMiss: nil,
+		},
+		{
+			name:         "job-level var overrides pipeline var",
+			pipelineVars: map[string]any{"FLAG": "false"},
+			jobs: map[string]model.Job{
+				"my-job": makeJob(map[string]any{"FLAG": "true"}, []model.Rule{
+					{If: `$FLAG == "true"`, When: "on_success"},
+				}),
+			},
+			// Job-level FLAG="true" makes the if: evaluate to true → no finding.
+			wantHit:  nil,
+			wantMiss: []string{"my-job"},
+		},
+		{
+			name:         "no rules — nothing to check",
+			pipelineVars: map[string]any{"DEPLOY": "false"},
+			jobs: map[string]model.Job{
+				"my-job": makeJob(nil, nil),
+			},
+			wantHit:  nil,
+			wantMiss: []string{"my-job"},
+		},
+		{
+			name:         "boolean variable evaluates correctly",
+			pipelineVars: map[string]any{"FLAG": false}, // bool false
+			jobs: map[string]model.Job{
+				"my-job": makeJob(nil, []model.Rule{
+					{If: `$FLAG == "true"`, When: "on_success"},
+				}),
+			},
+			// ScalarString(false) → "false"; "false" == "true" → false → GL042 fires.
+			wantHit:  []string{"my-job"},
+			wantMiss: nil,
+		},
+	}
+
+	for _, tc := range cases {
+		t.Run(tc.name, func(t *testing.T) {
+			p := &model.Pipeline{
+				Variables: tc.pipelineVars,
+				Jobs:      tc.jobs,
+			}
+			findings := checkRulesIfReachability(p)
+
+			hitSet := make(map[string]bool)
+			for _, f := range findings {
+				if f.Rule == RuleStaticDeadRules {
+					hitSet[f.Job] = true
+				}
+			}
+
+			for _, want := range tc.wantHit {
+				if !hitSet[want] {
+					t.Errorf("expected GL042 for job %q but it was not reported; findings: %v", want, findings)
+				}
+			}
+			for _, notWant := range tc.wantMiss {
+				if hitSet[notWant] {
+					t.Errorf("unexpected GL042 for job %q", notWant)
+				}
+			}
+		})
+	}
+}