Add root .prettierrc pinning prettier settings (closes #197)

Add a root `.prettierrc` (JSON) with `tabWidth: 4` and
`proseWrap: always`, the shared house style, so prettier formatting no
longer depends on host prettier defaults. Drop the now-redundant inline
`--tab-width 4` from `script/fmt` so the config file is the single
source of truth; `script/fmt` still formats only `static/js/*.js` and
`.prettierignore` still keeps the vendored `alpine.min.js` out.

Verified `make fmt` on a clean tree is a no-op: all first-party JS
reports unchanged and the vendored bundle is untouched.

Model: opus-4-8

.prettierrc

@@ -0,0 +1,4 @@
+{
+    "tabWidth": 4,
+    "proseWrap": "always"
+}

TODO.md

@@ -20,6 +20,10 @@ main cannot regress.
 
 # Completed Steps
 
+- 2026-09-22: Added a root `.prettierrc` (`tabWidth: 4`,
+  `proseWrap: always`) pinning the project prettier settings, and
+  dropped the now-redundant inline `--tab-width 4` from `script/fmt` so
+  the config file is the single source of truth (#197).
 - 2026-09-22: Fixed the flaky `t.TempDir` cleanup race in
   `internal/service/webhook` by tracking the async deployment goroutine
   in a `sync.WaitGroup` and exposing `WaitForDeployments`; tests now

script/fmt

@@ -8,7 +8,7 @@ main() {
     cd "$ROOT"
     gofmt -s -w .
     goimports -w .
-    npx prettier --write --tab-width 4 static/js/*.js
+    npx prettier --write static/js/*.js
 }
 
 main "$@"