feat(transport): swap Anthropic → opencode; add persona switcher

Part 1 — Transport swap:
- Replace anthropic.AsyncAnthropic streaming with httpx SSE client
  calling opencode's OpenAI-compat /v1/chat/completions on sin:4096
- Auth: basic auth opencode:$OPENCODE_PASSWORD
- Env: OPENCODE_URL (default http://sin:4096), OPENCODE_PASSWORD
- Sidecar binding (sin:4098) consulted per message to resolve active
  persona; voice read from binding → cart → env default
- Helper _session_id_for_user: deterministic sha256 slug per email
  so sidecar binding survives WebSocket reconnects
- anthropic dep retained in pyproject.toml (not removed — P4 may use it)

Part 2 — Persona switcher:
- PERSONAS dict: bt7274, friday, samantha (slug → voice/backend/prompt)
- POST /api/persona  — bind persona via sidecar, maps slug → full config
- GET  /api/persona/current — return current binding
- GET  /api/personas — list available personas
- chat.html: persona <select> in topnav with server-rendered active state
- chat.js: onChange → fetch /api/persona, update __personaName +
  status badge + system message in conversation feed

TODO: add CSS polish for .topnav__persona-wrap (inherits base styles for now)

app/templates/chat.html

@@ -23,10 +23,28 @@
   <script>
     window.__pilotName = {{ pilot_name | tojson }};
     window.__personaName = {{ persona_name | tojson }};
+    window.__personas = {{ personas | tojson }};
+    window.__boundSlug = {{ bound_slug | tojson }};
   </script>
   <nav class="topnav" aria-label="channel controls">
     <button type="button" class="topnav__link" id="recalibrate-btn" title="reset calibration">recalibrate</button>
     <span class="topnav__sep">·</span>
+
+    <!-- Persona switcher -->
+    <span class="topnav__persona-wrap">
+      <label class="topnav__persona-label" for="persona-select">persona</label>
+      <select class="topnav__persona-select" id="persona-select" title="Switch persona">
+        <option value="">— default —</option>
+        {% for p in personas %}
+        <option value="{{ p.slug }}"{% if p.slug == bound_slug %} selected{% endif %}>{{ p.display }}</option>
+        {% endfor %}
+      </select>
+      <span class="topnav__persona-status" id="persona-status">
+        {% if bound_display %}{{ bound_display }}{% endif %}
+      </span>
+    </span>
+    <span class="topnav__sep">·</span>
+
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