surface rate_limited status as red badge on per-agent page and dashboard

- add rate_limited: Arc<AtomicBool> to Bus; set/cleared by emit_status
- write/remove sentinel file hyperhive-rate-limited in state dir so host-side
  dashboard can detect it without a live socket call
- api_state returns status=rate_limited when flag is set (cold-load accurate)
- ALIVE_LABELS gains rate_limited entry (⊘ red chip) on per-agent page
- ContainerView gains rate_limited: bool read from sentinel file
- dashboard container row shows ⊘ rate limited badge (red) ahead of needs_login

Closes #24

hive-ag3nt/assets/app.js

@@ -547,8 +547,9 @@
   // class. Lives in the state row so the operator sees boot/login/
   // online without losing terminal real-estate to a paragraph.
   const ALIVE_LABELS = {
-    loading:                  { glyph: '…', text: 'connecting', cls: 'status-loading' },
-    online:                   { glyph: '●', text: 'alive',      cls: 'status-online' },
+    loading:                  { glyph: '…', text: 'connecting',  cls: 'status-loading' },
+    online:                   { glyph: '●', text: 'alive',       cls: 'status-online' },
+    rate_limited:             { glyph: '⊘', text: 'rate limited', cls: 'status-rate-limited' },
     needs_login_idle:         { glyph: '◌', text: 'needs login', cls: 'status-needs-login' },
     needs_login_in_progress:  { glyph: '◌', text: 'logging in',  cls: 'status-needs-login' },
     offline:                  { glyph: '○', text: 'offline',     cls: 'status-offline' },