Fix navigateTree API: add missing type updates, handler passthrough, and docs

- Update ExtensionCommandContext.navigateTree type signature
- Pass new options through in print-mode and rpc-mode handlers
- Update docs/extensions.md, docs/sdk.md, docs/tree.md
- Add changelog entry

packages/coding-agent/docs/extensions.md

@@ -733,9 +733,18 @@ Navigate to a different point in the session tree:
 ```typescript
 const result = await ctx.navigateTree("entry-id-456", {
   summarize: true,
+  customInstructions: "Focus on error handling changes",
+  replaceInstructions: false, // true = replace default prompt entirely
+  label: "review-checkpoint",
 });
 ```
 
+Options:
+- `summarize`: Whether to generate a summary of the abandoned branch
+- `customInstructions`: Custom instructions for the summarizer
+- `replaceInstructions`: If true, `customInstructions` replaces the default prompt instead of being appended
+- `label`: Label to attach to the branch summary entry (or target entry if not summarizing)
+
 ## ExtensionAPI Methods
 
 ### pi.on(event, handler)

packages/coding-agent/docs/sdk.md

@@ -110,7 +110,7 @@ interface AgentSession {
   
   // 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
+  navigateTree(targetId: string, options?: { summarize?: boolean; customInstructions?: string; replaceInstructions?: boolean; label?: string }): Promise<{ editorText?: string; cancelled: boolean }>;  // In-place navigation
   
   // Hook message injection
   sendHookMessage(message: HookMessage, triggerTurn?: boolean): Promise<void>;

packages/coding-agent/docs/tree.md

@@ -110,10 +110,21 @@ interface BranchSummaryEntry {
 ```typescript
 async navigateTree(
   targetId: string,
-  options?: { summarize?: boolean; customInstructions?: string }
+  options?: {
+    summarize?: boolean;
+    customInstructions?: string;
+    replaceInstructions?: boolean;
+    label?: string;
+  }
 ): Promise<{ editorText?: string; cancelled: boolean }>
 ```
 
+Options:
+- `summarize`: Whether to generate a summary of the abandoned branch
+- `customInstructions`: Custom instructions for the summarizer
+- `replaceInstructions`: If true, `customInstructions` replaces the default prompt instead of being appended
+- `label`: Label to attach to the branch summary entry (or target entry if not summarizing)
+
 Flow:
 1. Validate target, check no-op (target === current leaf)
 2. Find common ancestor between old leaf and target
@@ -153,21 +164,28 @@ interface TreePreparation {
   commonAncestorId: string | null;
   entriesToSummarize: SessionEntry[];
   userWantsSummary: boolean;
+  customInstructions?: string;
+  replaceInstructions?: boolean;
+  label?: string;
 }
 
 interface SessionBeforeTreeEvent {
   type: "session_before_tree";
   preparation: TreePreparation;
-  model: Model;
   signal: AbortSignal;
 }
 
 interface SessionBeforeTreeResult {
   cancel?: boolean;
   summary?: { summary: string; details?: unknown };
+  customInstructions?: string;    // Override custom instructions
+  replaceInstructions?: boolean;  // Override replace mode
+  label?: string;                 // Override label
 }
 ```
 
+Extensions can override `customInstructions`, `replaceInstructions`, and `label` by returning them from the `session_before_tree` handler.
+
 ### `session_tree`
 
 ```typescript