Consolidate session events: remove session_before_new/session_new, add reason field to switch events

- Remove session_before_new and session_new hook events
- Add reason: 'new' | 'resume' to session_before_switch and session_switch events
- Remove 'new' reason from custom tool onSession (use 'switch' for both /new and /resume)
- Rename reset() to newSession(options?) in AgentSession
- Add NewSessionOptions with optional parentSession for lineage tracking
- Rename branchedFrom to parentSession in SessionHeader
- Rename RPC reset command to new_session with optional parentSession
- Update example hooks to use new event structure
- Update documentation and changelog

Based on discussion in #293

packages/coding-agent/examples/hooks/dirty-repo-guard.ts

@@ -41,12 +41,9 @@ async function checkDirtyRepo(pi: HookAPI, ctx: HookContext, action: string): Pr
 }
 
 export default function (pi: HookAPI) {
-	pi.on("session_before_new", async (_event, ctx) => {
-		return checkDirtyRepo(pi, ctx, "new session");
-	});
-
-	pi.on("session_before_switch", async (_event, ctx) => {
-		return checkDirtyRepo(pi, ctx, "switch session");
+	pi.on("session_before_switch", async (event, ctx) => {
+		const action = event.reason === "new" ? "new session" : "switch session";
+		return checkDirtyRepo(pi, ctx, action);
 	});
 
 	pi.on("session_before_branch", async (_event, ctx) => {