[arnav] remove 6 dead HTTP endpoints

Verified each had no production caller (frontend, electron, or
internal backend) — only test references kept them looking alive
to coverage tools. Drops ~140 LOC of route handlers + helpers.
- POST /api/outputs/vibe-code: never wired up; frontend has no
  vibe-code UI. Also drops VibeCodeRequest model and
  VIBE_CODE_SYSTEM_PROMPT.
- GET /api/service/cost-breakdown: frontend Usage page reads
  /usage-summary, which already returns by_model/by_provider.
- GET /api/service/status: zero callers; was placeholder.
- GET /api/service/spool/count: debug-only, no UI surface.
- GET /api/settings/default-system-prompt: frontend defines its
  own DEFAULT_SYSTEM_PROMPT in settingsSlice.ts and never fetches
  the backend constant.
- POST /api/browser/command: sole caller (browser_mcp_server.py
  subprocess) was deleted in 8286cc1; browser_agent.py now calls
  ws_manager.send_browser_command directly in-process.
Tests covering the removed endpoints are dropped along with the
now-unused mock-anthropic helpers in test_api_outputs.py.

backend/tests/test_api_agents_subscriptions.py

@@ -15,7 +15,6 @@ chatter from these tests. Two fixtures here:
 Coverage:
 
   - GET  /subscriptions/status   — both down + up shapes.
-  - GET  /subscriptions/models   — both down + up shapes.
   - POST /subscriptions/connect  — missing provider → 400; 503 when
                                    9Router is unavailable; happy path
                                    round-trips the device_code flow.
@@ -126,25 +125,6 @@ def test_status_when_9router_up(client, nine_router_up):
     assert isinstance(body["models"], list)
 
 
-# ---------------------------------------------------------------------------
-# GET /subscriptions/models
-# ---------------------------------------------------------------------------
-
-
-def test_models_when_9router_down(client, nine_router_down):
-    resp = client.get("/api/agents/subscriptions/models")
-    assert resp.status_code == 200
-    assert resp.json() == {"models": []}
-
-
-def test_models_when_9router_up(client, nine_router_up):
-    resp = client.get("/api/agents/subscriptions/models")
-    assert resp.status_code == 200
-    body = resp.json()
-    assert isinstance(body["models"], list)
-    assert len(body["models"]) == 1
-
-
 # ---------------------------------------------------------------------------
 # POST /subscriptions/connect
 # ---------------------------------------------------------------------------