feat(frontend): per-agent effort-level quick-picker (#1596)

Adds a reasoning-effort quick-picker to the agent page's overflow menu,
a direct sibling of the existing model quick-picker, wired to the backend
seam from #1597/#1600: /api/state carries effort + available_efforts, the
picker POSTs { effort } to /api/effort (operator-only, mirroring
/api/model), and live updates arrive via the effort_changed SSE event.

The available levels are the backend's to declare — the frontend holds no
hard-coded list; availableEfforts is seeded from state.available_efforts
on cold-load and the picker section is omitted until the backend supplies
the set. Mirrors the model picker otherwise: postEffort(),
renderEffortChip() (updates the new amber effort-chip + button active
states), the cold-load seed, and the effort_changed handler. Build green.

Applies on the next claude session (operator uses the existing
clear-session), per the backend contract.

frontend/packages/agent/src/index.html

@@ -38,6 +38,7 @@
         <span id="alive-badge" class="status-badge status-loading" title="harness reachability">…</span>
         <span id="state-badge" class="state-badge state-loading">… booting</span>
         <span id="model-chip" class="model-chip" hidden></span>
+        <span id="effort-chip" class="effort-chip" hidden></span>
         <span id="ctx-badge" class="ctx-badge" hidden title="tokens used in the current context window"></span>
         <span id="cost-badge" class="ctx-badge" hidden title="cumulative tokens billed across the last turn (sum across every inference; tool-heavy turns rebill the cached prompt per call)"></span>
         <span id="last-turn" class="last-turn" hidden></span>