fix(coding-agent): preserve active operation UI during tree navigation

Follow-up to e687434a6.

Recheck compaction state after navigation dialogs and response cancellation before replacing status indicators or escape handlers. Document navigation conflicts and add regression coverage.

packages/coding-agent/docs/extensions.md

@@ -1203,7 +1203,7 @@ Options:
 
 ### ctx.navigateTree(targetId, options?)
 
-Navigate to a different point in the session tree:
+Navigate to a different point in the session tree. Rejects while an agent response, manual or automatic compaction, or another tree navigation is active, even with `summarize: false`. These conflicts leave the active branch unchanged and reject the promise rather than returning `{ cancelled: true }`. Wait for the active operation to finish (for example, with `await ctx.waitForIdle()` in a command handler) and retry:
 
 ```typescript
 const result = await ctx.navigateTree("entry-id-456", {

packages/coding-agent/docs/sdk.md

@@ -111,6 +111,8 @@ interface AgentSession {
 }
 ```
 
+`session.navigateTree()` rejects while an agent response, manual or automatic compaction, or another tree navigation is active, even with `summarize: false`. It does not queue navigation or return `{ cancelled: true }` for these conflicts. Wait for the active operation to finish (for example, with `await session.waitForIdle()`) and retry. Rejection leaves the active branch unchanged.
+
 Session replacement APIs such as new-session, resume, fork, and import live on `AgentSessionRuntime`, not on `AgentSession`.
 
 ### createAgentSessionRuntime() and AgentSessionRuntime