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.

docs/metrics/maintainability-baseline.json

@@ -1,9 +0,0 @@
-{
-  "sourceDirectory": "src",
-  "largeFileThresholdLoc": 350,
-  "typeScriptFileCount": 390,
-  "locInSrc": 83786,
-  "processExitReferenceCount": 185,
-  "synchronousFsApiReferenceCount": 894,
-  "largeFileCountOver350Loc": 61
-}