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/inherit_completeness.go

@@ -0,0 +1,112 @@
+package linter
+
+import (
+	"fmt"
+	"strings"
+
+	"git.k3nny.fr/glint/internal/model"
+)
+
+// checkInheritCompleteness (GL043): warn when a job's 'inherit: default:'
+// declaration is dead — either because there is no 'default:' block in the
+// pipeline, or because the list form references fields that are not defined
+// in the 'default:' block.
+func checkInheritCompleteness(p *model.Pipeline) []Finding {
+	var findings []Finding
+	for name, job := range p.Jobs {
+		findings = append(findings, checkJobInheritCompleteness(p, name, job)...)
+	}
+	return findings
+}
+
+func checkJobInheritCompleteness(p *model.Pipeline, name string, job model.Job) []Finding {
+	if job.Inherit == nil {
+		return nil
+	}
+	m, ok := job.Inherit.(map[string]any)
+	if !ok {
+		return nil
+	}
+	defaultVal, hasDefault := m["default"]
+	if !hasDefault {
+		return nil
+	}
+
+	// Case 1: no default: block at all — the entire declaration is a no-op.
+	if p.Default == nil {
+		return []Finding{{
+			Severity: Warning,
+			Rule:     RuleInheritNoDefault,
+			Job:      name,
+			File:     job.File,
+			Line:     job.Line,
+			Message:  "'inherit: default:' is declared but the pipeline has no 'default:' block — declaration has no effect",
+		}}
+	}
+
+	// Case 2: list form — check for field names not set in default:.
+	list, ok := defaultVal.([]any)
+	if !ok {
+		// bool form (true/false) with a non-nil default: block is always valid.
+		return nil
+	}
+
+	var dead []string
+	for _, item := range list {
+		field, ok := item.(string)
+		if !ok {
+			continue
+		}
+		if !defaultBlockHasField(p.Default, field) {
+			dead = append(dead, field)
+		}
+	}
+	if len(dead) == 0 {
+		return nil
+	}
+	return []Finding{{
+		Severity: Warning,
+		Rule:     RuleInheritNoDefault,
+		Job:      name,
+		File:     job.File,
+		Line:     job.Line,
+		Message: fmt.Sprintf(
+			"'inherit: default: [%s]': %s not defined in the 'default:' block — %s",
+			strings.Join(dead, ", "),
+			pluralIs(len(dead)),
+			"these entries have no effect",
+		),
+	}}
+}
+
+// defaultBlockHasField reports whether the given field name has a non-zero value
+// in the default: block. Unknown field names return true (conservative).
+func defaultBlockHasField(d *model.DefaultConfig, field string) bool {
+	switch field {
+	case "image":
+		return d.Image != nil
+	case "before_script":
+		return d.BeforeScript != nil
+	case "after_script":
+		return d.AfterScript != nil
+	case "cache":
+		return d.Cache != nil
+	case "artifacts":
+		return d.Artifacts != nil
+	case "retry":
+		return d.Retry != nil
+	case "timeout":
+		return d.Timeout != ""
+	case "tags":
+		return len(d.Tags) > 0
+	}
+	// Unknown field name — conservatively assume it might be defined.
+	return true
+}
+
+func pluralIs(n int) string {
+	if n == 1 {
+		return "this field is"
+	}
+	return "these fields are"
+}