✨ Add npm run fix aggregator for the fix:* scripts

Override the prior design choice: there is now an npm run fix
script that runs fix:oxlint && fix:oxfmt. Rationale: the friction
of having to remember and run two separate fix commands after
npm run check outweighs the 'intentional fixes' argument once
check has already told you which fixers are needed. The diff
after running fix remains the review surface.

- package.json: new 'fix' script (npm run fix:oxlint && npm run fix:oxfmt)
- project-specs.md: updated the FIX section to document the new
  aggregator, removed the 'no fix aggregator by design' text in
  two places (FIX section and PREFIX CONVENTION section)
- README.md: same updates, plus the Development section header
  changed from 'Format/Fix' to 'Individual fixes' to reflect
  the new hierarchy

package.json

@@ -43,6 +43,7 @@
         "check:oxlint": "oxlint ${LEFTHOOK_FILES:-src}",
         "check:tsc": "tsc",
         "clean": "node -e \"fs.rmSync('dist', { recursive: true, force: true })\"",
+        "fix": "npm run fix:oxlint && npm run fix:oxfmt",
         "fix:oxfmt": "oxfmt ${LEFTHOOK_FILES:-.}",
         "fix:oxlint": "oxlint --fix src",
         "test": "npm run check:tsc && node --test --strip-types \"src/**/*.test.ts\"",