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/components/StatusChips.css

@@ -0,0 +1,16 @@
+/* <StatusChips> layout. Colours all come from Badge/Dropdown's own CSS
+   (@hive/shared) — nothing here overrides a base16 slot. */
+.status-chips {
+  display: flex;
+  flex-wrap: wrap;
+  align-items: center;
+  gap: 0.5em;
+}
+.status-chip-anchor {
+  position: relative;
+  display: inline-block;
+}
+.status-chips-last-turn {
+  font-size: 0.8em;
+  color: var(--muted);
+}