✨ Add knip for unused dependency and dead code detection

- Add check:knip using --include dependencies,exports,files
  (skips the noisy 'types' category, which produces false positives
  for libraries whose exported types are part of the public API)
- Remove tslib and type-fest (both caught as unused by knip)
- Add 'knip' to cspell word list
- No knip config file: the --include flag keeps the scope targeted
  without boilerplate, matching the 'keep it simple' principle
- Document in project-specs.md (CHECK section) and README.md

project-specs.md

@@ -217,7 +217,7 @@ script does not belong in the standard pipeline.
 ### CHECK
 
 - `check`: Run all checks in order — `check:oxlint`, `check:oxfmt`,
-  `check:tsc`, `check:cspell`, `check:outdated`.
+  `check:tsc`, `check:cspell`, `check:knip`, `check:outdated`.
 - `check:oxlint`: `oxlint ${LEFTHOOK_FILES:-src}` — lints `src/`
   by default; when invoked from the lefthook pre-commit hook with
   `LEFTHOOK_FILES` set to the staged-files list, lints only those
@@ -239,6 +239,14 @@ script does not belong in the standard pipeline.
 - `check:cspell`: `cspell lint ${LEFTHOOK_FILES:-.}` — walks the
   project root by default; from lefthook, only the staged files.
 - `check:outdated`: `check-outdated --ignore-pre-releases --ignore-packages @oxfmt/binding-*,@oxlint/binding-*`. The oxc native bindings are declared as `optionalDependencies` so the correct one is selected per platform automatically; the `*`-platform bindings show as "not installed" on the current platform and are explicitly ignored here.
+- `check:knip`: `knip --include dependencies,exports,files` — finds
+  unused dependencies, value exports, and source files. The
+  scoped `--include` list intentionally omits the `types`
+  category, which produces systematic false positives for
+  libraries whose exported types (e.g. `Matcher`, `Pattern`) are
+  part of the public API and not consumed internally. The
+  targeted scope keeps the signal high (real unused-dep
+  detection) without config-file boilerplate.
 
 ### FIX