Add before/after session events with cancellation support

- Merge branch event into session with before_branch/branch reasons
- Add before_switch, before_clear, shutdown reasons
- before_* events can be cancelled with { cancel: true }
- Update RPC commands to return cancelled status
- Add shutdown event on process exit
- New example hooks: confirm-destructive, dirty-repo-guard, auto-commit-on-exit

fixes #278

packages/coding-agent/test/agent-session-branching.test.ts

@@ -86,7 +86,7 @@ describe.skipIf(!API_KEY)("AgentSession branching", () => {
 		// Branch from the first message
 		const result = await session.branch(userMessages[0].entryIndex);
 		expect(result.selectedText).toBe("Say hello");
-		expect(result.skipped).toBe(false);
+		expect(result.cancelled).toBe(false);
 
 		// After branching, conversation should be empty (branched before the first message)
 		expect(session.messages.length).toBe(0);
@@ -116,7 +116,7 @@ describe.skipIf(!API_KEY)("AgentSession branching", () => {
 		// Branch from the first message
 		const result = await session.branch(userMessages[0].entryIndex);
 		expect(result.selectedText).toBe("Say hi");
-		expect(result.skipped).toBe(false);
+		expect(result.cancelled).toBe(false);
 
 		// After branching, conversation should be empty
 		expect(session.messages.length).toBe(0);