Rename /branch command to /fork

- RPC: branch -> fork, get_branch_messages -> get_fork_messages
- SDK: branch() -> fork(), getBranchMessages() -> getForkMessages()
- AgentSession: branch() -> fork(), getUserMessagesForBranching() -> getUserMessagesForForking()
- Extension events: session_before_branch -> session_before_fork, session_branch -> session_fork
- Settings: doubleEscapeAction 'branch' -> 'fork'

fixes #641

packages/coding-agent/docs/extensions.md

@@ -278,9 +278,9 @@ user sends another prompt ◄─────────────────
   ├─► session_before_switch (can cancel)
   └─► session_switch
 
-/branch
-  ├─► session_before_branch (can cancel)
-  └─► session_branch
+/fork
+  ├─► session_before_fork (can cancel)
+  └─► session_fork
 
 /compact or auto-compaction
   ├─► session_before_compact (can cancel or customize)
@@ -334,19 +334,19 @@ pi.on("session_switch", async (event, ctx) => {
 
 **Examples:** [confirm-destructive.ts](../examples/extensions/confirm-destructive.ts), [dirty-repo-guard.ts](../examples/extensions/dirty-repo-guard.ts), [status-line.ts](../examples/extensions/status-line.ts), [todo.ts](../examples/extensions/todo.ts)
 
-#### session_before_branch / session_branch
+#### session_before_fork / session_fork
 
-Fired when branching via `/branch`.
+Fired when forking via `/fork`.
 
 ```typescript
-pi.on("session_before_branch", async (event, ctx) => {
-  // event.entryId - ID of the entry being branched from
-  return { cancel: true }; // Cancel branch
+pi.on("session_before_fork", async (event, ctx) => {
+  // event.entryId - ID of the entry being forked from
+  return { cancel: true }; // Cancel fork
   // OR
-  return { skipConversationRestore: true }; // Branch but don't rewind messages
+  return { skipConversationRestore: true }; // Fork but don't rewind messages
 });
 
-pi.on("session_branch", async (event, ctx) => {
+pi.on("session_fork", async (event, ctx) => {
   // event.previousSessionFile - previous session file
 });
 ```

packages/coding-agent/docs/rpc.md

@@ -541,47 +541,47 @@ If a hook cancelled the switch:
 {"type": "response", "command": "switch_session", "success": true, "data": {"cancelled": true}}
 ```
 
-#### branch
+#### fork
 
-Create a new branch from a previous user message. Can be cancelled by a `before_branch` hook. Returns the text of the message being branched from.
+Create a new fork from a previous user message. Can be cancelled by a `before_fork` hook. Returns the text of the message being forked from.
 
 ```json
-{"type": "branch", "entryId": "abc123"}
+{"type": "fork", "entryId": "abc123"}
 ```
 
 Response:
 ```json
 {
   "type": "response",
-  "command": "branch",
+  "command": "fork",
   "success": true,
   "data": {"text": "The original prompt text...", "cancelled": false}
 }
 ```
 
-If a hook cancelled the branch:
+If a hook cancelled the fork:
 ```json
 {
   "type": "response",
-  "command": "branch",
+  "command": "fork",
   "success": true,
   "data": {"text": "The original prompt text...", "cancelled": true}
 }
 ```
 
-#### get_branch_messages
+#### get_fork_messages
 
-Get user messages available for branching.
+Get user messages available for forking.
 
 ```json
-{"type": "get_branch_messages"}
+{"type": "get_fork_messages"}
 ```
 
 Response:
 ```json
 {
   "type": "response",
-  "command": "get_branch_messages",
+  "command": "get_fork_messages",
   "success": true,
   "data": {
     "messages": [

packages/coding-agent/docs/sdk.md

@@ -108,8 +108,8 @@ interface AgentSession {
   newSession(options?: { parentSession?: string }): Promise<boolean>;  // Returns false if cancelled by hook
   switchSession(sessionPath: string): Promise<boolean>;
   
-  // Branching
-  branch(entryId: string): Promise<{ selectedText: string; cancelled: boolean }>;  // Creates new session file
+  // Forking
+  fork(entryId: string): Promise<{ selectedText: string; cancelled: boolean }>;  // Creates new session file
   navigateTree(targetId: string, options?: { summarize?: boolean }): Promise<{ editorText?: string; cancelled: boolean }>;  // In-place navigation
   
   // Hook message injection

packages/coding-agent/docs/session.md

@@ -49,7 +49,7 @@ First line of the file. Metadata only, not part of the tree (no `id`/`parentId`)
 {"type":"session","version":3,"id":"uuid","timestamp":"2024-12-03T14:00:00.000Z","cwd":"/path/to/project"}
 ```
 
-For sessions with a parent (created via `/branch` or `newSession({ parentSession })`):
+For sessions with a parent (created via `/fork` or `newSession({ parentSession })`):
 
 ```json
 {"type":"session","version":3,"id":"uuid","timestamp":"2024-12-03T14:00:00.000Z","cwd":"/path/to/project","parentSession":"/path/to/original/session.jsonl"}

packages/coding-agent/docs/tree.md

@@ -6,14 +6,14 @@ The `/tree` command provides tree-based navigation of the session history.
 
 Sessions are stored as trees where each entry has an `id` and `parentId`. The "leaf" pointer tracks the current position. `/tree` lets you navigate to any point and optionally summarize the branch you're leaving.
 
-### Comparison with `/branch`
+### Comparison with `/fork`
 
-| Feature | `/branch` | `/tree` |
-|---------|-----------|---------|
+| Feature | `/fork` | `/tree` |
+|---------|---------|---------|
 | View | Flat list of user messages | Full tree structure |
 | Action | Extracts path to **new session file** | Changes leaf in **same session** |
 | Summary | Never | Optional (user prompted) |
-| Events | `session_before_branch` / `session_branch` | `session_before_tree` / `session_tree` |
+| Events | `session_before_fork` / `session_fork` | `session_before_tree` / `session_tree` |
 
 ## Tree UI