Enable noUncheckedIndexedAccess in tsconfig

Tighter type-checking: array indexing and dynamic property access now
return T | undefined rather than T. Catches bugs where code assumes a
key exists without checking.

No code changes were needed — the existing modules already either:
  - cast through Record<string, unknown> (configFile.ts), where access
    is already 'unknown';
  - cast values from parseArgs to '... | undefined' (cli.ts), already
    nullable; or
  - use static, statically-known fields (Config, ConfigOverrides,
    DEFAULT_CONFIG).

tsc --noEmit remains clean. The full test suite still passes.

tsconfig.json

@@ -9,7 +9,8 @@
     "noEmit": true,
     "allowImportingTsExtensions": true,
     "verbatimModuleSyntax": true,
-    "resolveJsonModule": true
+    "resolveJsonModule": true,
+    "noUncheckedIndexedAccess": true
   },
   "include": ["src/**/*.ts"]
 }