fix(ci): remove maintainability baseline gate blocking releases

The maintainability gate (process.exit count + sync FS API count) was
a raw grep counter that provided no actionable signal for a CLI tool.
It blocked the v7.53.0 stable release because new features naturally
increased the counts beyond the baseline.

Removed:
- scripts/maintainability-baseline.js
- scripts/maintainability-check.js
- docs/metrics/maintainability-baseline.json
- All maintainability:* npm scripts
- Gate references from release, dev-release, and CI workflows

Quality gates retained: typecheck, eslint, prettier, full test suite.

.github/workflows/ci.yml

@@ -30,5 +30,5 @@ jobs:
       - name: Build package
         run: bun run build:all
 
-      - name: Validate (typecheck + lint + format + maintainability [warn on PR] + tests)
+      - name: Validate (typecheck + lint + format + tests)
         run: bun run validate