agent: scaffold Preact rewrite, Header + StatusChips first slice

Adds a new Preact/TSX build alongside the existing app.js (esbuild
entry `main.tsx` → dist/static/main.{js,css}, same jsx/tsconfig shape
swarm-ui already uses) and the first real page slice: Header +
StatusChips, composing the Badge/Dropdown components from the prior
commit's PR. Not wired into index.html yet — app.js keeps rendering
the live page untouched while this fills in component by component
(state polling, the live SSE stream, login flow, inbox/todos, term
input) in follow-up commits on this branch.

StatusChips folds the model/effort pickers and pause into the badge
row itself (each badge IS its own control), replacing the old
overflow-menu-only pickers — the concrete fix the design guide already
names this page as needing. Presentational only for now (props, not
live data) so it's reviewable against sample data before being wired
to /api/state.

Screenshot-verified against the real agent.css/theme.css/colors.css
(headless chromium, sample data) — renders correctly.

Builds + tsc --noEmit clean.

frontend/packages/agent/build.mjs

@@ -47,6 +47,25 @@ 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.
+await build({
+  entryPoints: [src('main.tsx')],
+  outfile: staticDir('main.js'),
+  bundle: true,
+  format: 'esm',
+  platform: 'browser',
+  target: ['es2022'],
+  sourcemap: true,
+  logLevel: 'info',
+  jsx: 'automatic',
+  jsxImportSource: 'preact',
+  loader: { '.css': 'css' },
+});
+
 // Bundle the CSS. `colors.css` re-exports the standalone base16 palette
 // (the theme swap contract — its own output file so a swap replaces only
 // it, no bundle rebuild); `theme.css` is the semantic derivation layer;