feat(telegram): add per-chat/topic default agents (#109)

docs/specification.md

@@ -353,6 +353,15 @@ Decision (v0.4.0):
 * Users MAY prefix the first non-empty line with `/{engine}` (e.g. `/claude`, `/codex`, or `/pi`) to select the engine for a **new** thread.
   * The bridge MUST strip that directive from the prompt before invoking the runner.
   * If a ResumeToken is resolved from the message or reply, it MUST take precedence and the `/{engine}` directive MUST be ignored.
+* Bridges MAY persist default engine overrides per Telegram scope:
+  * **Topic default**: forum topic (`chat_id + thread_id`)
+  * **Chat default**: chat (`chat_id`)
+* When no ResumeToken is resolved, engine selection MUST follow this precedence:
+  1) explicit `/{engine}` directive
+  2) topic default (if any)
+  3) chat default (if any)
+  4) project default engine (if configured for the resolved context)
+  5) global default engine
 
 ### 8.1 Command menu (Telegram)
 

docs/user-guide.md

@@ -122,6 +122,22 @@ Prefix your message with an engine directive to override the default:
 
 Directives are only parsed at the start of the first non-empty line.
 
+### Default agent per chat or topic
+
+Use `/agent` to view or set a persistent default for the current scope:
+
+```
+/agent
+/agent set claude
+/agent clear
+```
+
+- Inside a forum topic, `/agent set` affects that topic.
+- In normal chats, it affects the whole chat.
+- In group chats, only admins can change defaults.
+
+Precedence (highest to lowest): resume token → `/engine` directive → topic default → chat default → project default → global default.
+
 ### Setting up engines
 
 Takopi shells out to the agent CLIs. Install them and make sure they're on your `PATH`
@@ -257,7 +273,7 @@ takopi chat-id
 
 ## 7. Topics
 
-Topics bind Telegram forum threads to specific project/branch contexts. They also preserve resume tokens, so agents can pick up where they left off.
+Topics bind Telegram forum threads to specific project/branch contexts. They also preserve resume tokens and can store a default agent per topic.
 
 ### Enabling topics
 
@@ -341,7 +357,7 @@ path = "~/dev/takopi"
 chat_id = -1001111111111   # forum-enabled group
 ```
 
-Topic state is stored in `telegram_topics_state.json` next to your config file.
+Topic state is stored in `telegram_topics_state.json` next to your config file. Chat defaults live in `telegram_chat_prefs_state.json`.
 
 ---
 
@@ -496,6 +512,7 @@ worktree_base = "develop"
 | `/cancel` | Reply to the progress message to stop the current run |
 | `/file put <path>` | Upload a document into the repo/worktree |
 | `/file get <path>` | Fetch a file (directories are zipped) |
+| `/agent` | Show/set the default agent for the current scope |
 | `/topic <project> @branch` | Create/bind a topic |
 | `/ctx` | Show current context |
 | `/ctx set <project> @branch` | Update context binding |