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>

Taskfile.yml

@@ -107,6 +107,18 @@ tasks:
         ignore_error: false
       - cmd: ./{{.BINARY}} check testdata/config_suppress/.gitlab-ci.yml
         ignore_error: false
+      - cmd: ./{{.BINARY}} check testdata/static_dead_rules.yml
+        ignore_error: false
+      - cmd: ./{{.BINARY}} check testdata/inherit_dead.yml
+        ignore_error: false
+      - cmd: ./{{.BINARY}} check testdata/inherit_dead_fields.yml
+        ignore_error: false
+      - cmd: ./{{.BINARY}} explain GL007
+        ignore_error: false
+      - cmd: ./{{.BINARY}} explain gl042
+        ignore_error: false
+      - cmd: ./{{.BINARY}} explain
+        ignore_error: false
 
   lint-go:
     desc: Run go vet on all packages