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/config.rs

@@ -288,6 +288,11 @@ pub struct BackendConfig {
     /// Context window size in tokens.
     #[serde(default, skip_serializing_if = "Option::is_none")]
     pub context_window: Option<usize>,
+    /// Use chat completions API (/v1/chat/completions) instead of
+    /// raw completions (/v1/completions). Required for cloud API
+    /// providers (OpenRouter, Anthropic, etc).
+    #[serde(default)]
+    pub chat_api: bool,
 }
 
 #[derive(Debug, Clone, Serialize, Deserialize)]
@@ -370,6 +375,8 @@ pub struct SessionConfig {
     pub app: AppConfig,
     /// Disable background agents (surface, observe, scoring)
     pub no_agents: bool,
+    /// Use chat completions API instead of raw completions.
+    pub chat_api: bool,
 }
 
 /// A fully resolved model ready to construct an ApiClient.
@@ -380,6 +387,7 @@ pub struct ResolvedModel {
     pub api_key: String,
     pub model_id: String,
     pub context_window: Option<usize>,
+    pub chat_api: bool,
 }
 
 impl AppConfig {
@@ -415,6 +423,7 @@ impl AppConfig {
             session_dir,
             app: self.clone(),
             no_agents: cli.no_agents,
+            chat_api: resolved.chat_api,
         })
     }
 
@@ -439,6 +448,7 @@ impl AppConfig {
             api_key: b.api_key.clone(),
             model_id: b.model_id.clone(),
             context_window: b.context_window,
+            chat_api: b.chat_api,
         })
     }