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/src/main.tsx

@@ -0,0 +1,16 @@
+// Entry point for the Preact rewrite of the per-agent terminal page
+// (replacing app.js's imperative DOM). Not wired into index.html yet —
+// this lands in small, reviewable slices (component by component) per
+// mara's "one pr != one commit" note on the issue; `index.html`'s
+// `<script>` tag swaps from `app.js` to this bundle's output only once
+// the new page covers everything the old one did (state polling, the
+// live SSE stream, login flow, inbox/todos flyouts, slash commands).
+//
+// Mounts to `#preact-root`, a sibling of the legacy markup rather than
+// a full document takeover, so the two can coexist on a branch while
+// this is still in progress.
+import { render } from 'preact';
+import { Root } from './Root.js';
+
+const root = document.getElementById('preact-root');
+if (root) render(<Root />, root);