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/rules.go

@@ -143,4 +143,15 @@ const (
 
 	// GL041: cache.key.files contains a glob pattern; it must be a list of exact file paths.
 	RuleInvalidCacheKeyFiles = "GL041"
+
+	// GL042: every rules:if: condition in a job's rules: block evaluates to false
+	// given the values of variables declared in the pipeline YAML, so the job can
+	// never be active. Only fires when all referenced variables are declared
+	// (predefined CI_* / GITLAB_* vars are not evaluated to avoid false positives).
+	RuleStaticDeadRules = "GL042"
+
+	// GL043: a job declares 'inherit: default:' (true, false, or list) but the
+	// pipeline has no 'default:' block, making the declaration a no-op. Also fires
+	// when 'inherit: default: [list]' names fields not set in the default: block.
+	RuleInheritNoDefault = "GL043"
 )