Add cloud API support and per-agent model override

Cloud API support:
- Add chat_api config flag to BackendConfig, threaded through
  SessionConfig → ResolvedModel → Agent → Mind
- New StreamToken::TextDelta variant for chat completions streaming
- stream_chat_completion() method on ApiClient: builds messages array,
  sends to /v1/chat/completions, parses SSE stream
- ChatMessage struct and wire_messages() on ContextState: converts the
  AST (system/identity/journal/conversation nodes) into a messages
  array for the chat API, handling images as base64 data URIs
- ResponseParser handles TextDelta alongside Token variants
- TUI rendering fix: tokens() returns byte-length estimate (~4
  bytes/token) when tokenizer isn't loaded, so the change detector
  actually triggers re-renders
- Gate all vLLM-specific scoring (memory scoring, finetune scoring,
  compare scoring) behind !chat_api checks

Per-agent model override:
- Add model field to agent definition headers (.agent files)
- Thread through AutoAgent → prepare_spawn → resolve_model
- Agents fall back to default_backend when model is unset
- Enables cheaper backends (e.g. Kimi) for graph maintenance agents
  while keeping Sonnet for conversation

Tested: end-to-end with Poe API + Haiku, chat_api: true in config.
TUI starts, messages send, responses stream and render.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/mind/subconscious.rs

@@ -357,6 +357,7 @@ impl SubconsciousAgent {
         let auto = AutoAgent::new(
             name.to_string(), tools, steps,
             def.temperature.unwrap_or(0.6), def.priority,
+            def.model.clone(),
         );
 
         Some(Self {