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>

testdata/inherit_dead.yml

@@ -0,0 +1,26 @@
+stages:
+  - build
+  - test
+
+# No default: block — any inherit: default: declaration is a no-op.
+
+# GL043: inherit: default: false but no default: block.
+isolated-job:
+  stage: build
+  inherit:
+    default: false
+  script: echo isolated
+
+# GL043: inherit: default: [list] but no default: block.
+selective-inherit-job:
+  stage: test
+  inherit:
+    default: [image, tags]
+  script: echo selective
+
+# Not GL043: inherit only touches variables: (no default: check needed).
+vars-inherit-job:
+  stage: test
+  inherit:
+    variables: false
+  script: echo vars-only