✨ 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

project-specs.md

@@ -176,9 +176,9 @@ script's intended lifecycle:
   (which runs all `check:*` scripts in order). Called from the
   lefthook pre-commit hook on staged files, and from the CI build
   job on the full project. Read-only; never modifies files.
-- `fix:*` — mutating counterpart of a `check:*` script. There is
-  **no** `npm run fix` aggregator by design: fixes should be
-  intentional, not batched. Run individually.
+- `fix:*` — mutating counterpart of a `check:*` script. Aggregated
+  by `npm run fix`. Use after `npm run check` to auto-resolve
+  issues; the diff is the review surface.
 - `test:*` — test scripts. `npm run test` is the canonical entry
   point (`check:tsc` + unit tests); `test:unit` skips the typecheck
   for fast local iteration; `test:ci` adds c8 coverage and is the
@@ -268,12 +268,14 @@ script does not belong in the standard pipeline.
 
 ### FIX
 
+- `fix`: Run all fixes in order — `fix:oxlint`, then `fix:oxfmt`.
+  Use this when `npm run check` reports issues you want to
+  auto-resolve. The fix scripts write changes in place; review
+  the diff before committing.
 - `fix:oxlint`: `oxlint --fix src`.
 - `fix:oxfmt`: `oxfmt ${LEFTHOOK_FILES:-.}` — same scoping as
   `check:oxfmt` (whole project by default, staged files from
   lefthook). Writes changes in place.
-  (There is no `fix` aggregator in the scripts; run the `fix:*` scripts
-  individually.)
 
 ### PUBLISH