Invoke the formatter directly from the format hook (#1594)

* Invoke the formatter directly from the format hook

The PostToolUse format hook now runs prettier directly with a pinned
version and --no-config instead of going through the package.json
"format" script, so the hook resolves the same way regardless of the
scripts and formatter config in the checked-out tree. Output matches
the previous "bun run format" (both .prettierrc files are empty).

Also documents which paths the action restores from the PR base branch
and recommends keeping base-branch hooks self-contained.

No-Verification-Needed: config, comment, and doc-only change

* Qualify the self-contained hook guidance for Bun-only runners

Note in docs/security.md and the restore-config JSDoc that bunx runs the
tool under node when node is on PATH, but on a Bun-only runner Bun runs
the script itself and reads bunfig.toml (preload etc.) from the
checkout, so that file and .npmrc are runtime config from the PR head.

No-Verification-Needed: comment- and doc-only change

* Exclude .claude-pr from prettier

No-Verification-Needed: prettierignore-only change

.claude/settings.json

@@ -5,7 +5,7 @@
         "hooks": [
           {
             "type": "command",
-            "command": "bun run format"
+            "command": "bunx prettier@3.5.3 --no-config --write ."
           }
         ],
         "matcher": "Edit|Write|MultiEdit"