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_fields.yml

@@ -0,0 +1,20 @@
+stages:
+  - build
+  - test
+
+default:
+  image: node:20
+
+# GL043: before_script is in the list but not defined in default:.
+selective-bad:
+  stage: build
+  inherit:
+    default: [image, before_script]   # before_script not in default:
+  script: echo hi
+
+# Not GL043: image IS defined in default: — list is valid.
+selective-good:
+  stage: test
+  inherit:
+    default: [image]
+  script: echo hi