Add session management and agent state methods to hooks API

HookAPI additions:
- pi.newSession(options?) - create new session with optional setup callback
- pi.branch(entryId) - branch from a specific entry
- pi.navigateTree(targetId, options?) - navigate the session tree

HookContext additions:
- ctx.isIdle() - check if agent is streaming
- ctx.waitForIdle() - wait for agent to finish
- ctx.abort() - abort current operation
- ctx.hasQueuedMessages() - check for queued user messages

These enable hooks to programmatically manage sessions (handoff, templates)
and check agent state before showing interactive UI.

Fixes #388

packages/coding-agent/test/compaction-hooks.test.ts

@@ -80,6 +80,9 @@ describe.skipIf(!API_KEY)("Compaction hooks", () => {
 			commands: new Map(),
 			setSendMessageHandler: () => {},
 			setAppendEntryHandler: () => {},
+			setNewSessionHandler: () => {},
+			setBranchHandler: () => {},
+			setNavigateTreeHandler: () => {},
 		};
 	}
 
@@ -266,6 +269,9 @@ describe.skipIf(!API_KEY)("Compaction hooks", () => {
 			commands: new Map(),
 			setSendMessageHandler: () => {},
 			setAppendEntryHandler: () => {},
+			setNewSessionHandler: () => {},
+			setBranchHandler: () => {},
+			setNavigateTreeHandler: () => {},
 		};
 
 		createSession([throwingHook]);
@@ -313,6 +319,9 @@ describe.skipIf(!API_KEY)("Compaction hooks", () => {
 			commands: new Map(),
 			setSendMessageHandler: () => {},
 			setAppendEntryHandler: () => {},
+			setNewSessionHandler: () => {},
+			setBranchHandler: () => {},
+			setNavigateTreeHandler: () => {},
 		};
 
 		const hook2: LoadedHook = {
@@ -342,6 +351,9 @@ describe.skipIf(!API_KEY)("Compaction hooks", () => {
 			commands: new Map(),
 			setSendMessageHandler: () => {},
 			setAppendEntryHandler: () => {},
+			setNewSessionHandler: () => {},
+			setBranchHandler: () => {},
+			setNavigateTreeHandler: () => {},
 		};
 
 		createSession([hook1, hook2]);