agent: cutover — index.html now serves the Preact rewrite, app.js deleted

mara: 'i merged this without realizing you still did not delete the
old code. pls follow up with ripping out the old code.'

- index.html: dropped the old static header/main/composer/overflow-
  menu markup and app.js's <script> tag. Now just <div id=preact-root>
  + <script src=static/main.js>. main.css's <link> lands after
  agent.css's — cascade order argus flagged matters for LoginFlow's
  .login-card / MetaNav's popover to win against agent.css's legacy
  rules, confirmed explicitly rather than assumed at this exact step.
- build.mjs: app.js dropped from the esbuild entryPoints (stats.js
  keeps its own bundle, unaffected — separate page, separate script).
- frontend/packages/agent/src/app.js deleted (1717 lines). screen.html
  has its own inline <script>, untouched — never depended on app.js.
- docs/web-ui/agent.md: rewrote the Header section to describe the
  real Preact component tree and the badges+pills-together layout
  (deferred from the earlier commits on this PR specifically so it
  wouldn't describe a hybrid state — this is that promised follow-up).
  Touched up the one other app.js-specific mention in the endpoints
  section. Left the Main/composer/side-panel/live-view/slash-command
  sections alone — behavior there is unchanged, verified faithfully
  ported throughout this PR's earlier commits.

Verified against the REAL dist/index.html (not the dev-preview
harness) — a scratch mock server serving the actual built output
end-to-end, screenshotted clean. tsc --noEmit clean, build clean, both
pre-push lints clean.

frontend/packages/agent/build.mjs

@@ -1,14 +1,18 @@
 // esbuild build for @hive/agent. Output layout (`dist/`):
 //
-//   dist/index.html            served at GET /
+//   dist/index.html            served at GET / (the Preact rewrite,
+//                               hyperhive#3685 — app.js is deleted)
 //   dist/stats.html            served at GET /stats
 //   dist/screen.html           served at GET /screen
-//   dist/static/app.js         served at /static/app.js (ESM bundle,
-//                               pulls in @hive/shared + marked)
-//   dist/static/app.js.map     source map sibling
 //   dist/static/stats.js       served at /static/stats.js (pulls in
 //                               chart.js/auto)
 //   dist/static/stats.js.map   source map sibling
+//   dist/static/main.js        served at /static/main.js (the Preact
+//                               bundle — see main.tsx)
+//   dist/static/main.js.map    source map sibling
+//   dist/static/main.css       served at /static/main.css (bundled
+//                               component CSS — see main.tsx's build
+//                               step below)
 //   dist/static/agent.css      served at /static/agent.css (@import
 //                               resolved from @hive/shared)
 //
@@ -30,10 +34,9 @@ const staticDir = (p) => resolve(here, 'dist', 'static', p);
 rmSync(dist(''), { recursive: true, force: true });
 mkdirSync(staticDir(''), { recursive: true });
 
-// Two JS entries: the main app + the stats page. Both bundle their
-// own deps so each page can be loaded independently.
+// The stats page's own bundle — chart.js/auto, its own deps.
 await build({
-  entryPoints: [src('app.js'), src('stats.js')],
+  entryPoints: [src('stats.js')],
   outdir: staticDir(''),
   bundle: true,
   format: 'esm',
@@ -47,11 +50,13 @@ await build({
   loader: { '.css': 'text' },
 });
 
-// The Preact rewrite (in progress — see main.tsx's file comment). A
-// third, separate bundle rather than folding into the `app.js` build
-// above: different loader needs (`.css: 'css'`, real stylesheets a
-// component imports, vs. `app.js`'s raw-text shadow-DOM CSS) and JSX
-// transpilation, same split swarm-ui's own build already makes.
+// The main agent page (hyperhive#3685's Preact rewrite — app.js,
+// which used to share this build step, is deleted). A separate bundle
+// from stats.js above rather than folded in: different loader needs
+// (`.css: 'css'`, real stylesheets a component imports, vs. the raw-
+// text shadow-DOM CSS `stats.js` still needs for `@hive/shared/
+// modal.js`) and JSX transpilation, same split swarm-ui's own build
+// already makes.
 await build({
   entryPoints: [src('main.tsx')],
   outfile: staticDir('main.js'),