check: cache prettier and eslint results

Both re-lint every file on every run. Measured on this repo: prettier
19.5s, eslint 108s. Neither result changes for a file that has not
changed, so both keep a cache keyed on content.

Warm: prettier 7.1s, eslint 2.4s -- a full lint goes from ~127s to
~4.3s. Cold cost is unchanged, so CI (fresh checkout, no cache) is
unaffected; this buys back the local edit-check loop.

Author: TheGreatAxios

.gitignore

@@ -31,3 +31,6 @@ tmp/
 
 # Browser walkthrough harness screenshots + run summary (CL-6072)
 scripts/e2e/browser/shots/
+
+# Lint caches — local only; CI installs fresh and rebuilds them.
+.eslintcache

package.json

@@ -14,7 +14,7 @@
     "check": "bun run typecheck && bun run lint && bun run test && bun run check:structural",
     "check:all": "bun run check && bun run check:packages",
     "typecheck": "bun run scripts/run-all.ts typecheck",
-    "lint": "prettier --check . && eslint .",
+    "lint": "prettier --check --cache . && eslint --cache .",
     "format": "prettier --write .",
     "build": "bun run scripts/run-all.ts build",
     "test": "bun test scripts/run-all.test.ts && bun run scripts/run-all.ts test",