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/status-line.ts

@@ -30,9 +30,11 @@ export default function (pi: HookAPI) {
 		ctx.ui.setStatus("status-demo", check + text);
 	});
 
-	pi.on("session_new", async (_event, ctx) => {
-		turnCount = 0;
-		const theme = ctx.ui.theme;
-		ctx.ui.setStatus("status-demo", theme.fg("dim", "Ready"));
+	pi.on("session_switch", async (event, ctx) => {
+		if (event.reason === "new") {
+			turnCount = 0;
+			const theme = ctx.ui.theme;
+			ctx.ui.setStatus("status-demo", theme.fg("dim", "Ready"));
+		}
 	});
 }