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/docs/hooks.md

@@ -123,13 +123,9 @@ user sends prompt ────────────────────
                                                            │
 user sends another prompt ◄────────────────────────────────┘
 
-/new (new session)
-  ├─► session_before_new (can cancel)
-  └─► session_new
-
-/resume (switch session)
-  ├─► session_before_switch (can cancel)
-  └─► session_switch
+/new (new session) or /resume (switch session)
+  ├─► session_before_switch (can cancel, has reason: "new" | "resume")
+  └─► session_switch (has reason: "new" | "resume")
 
 /branch
   ├─► session_before_branch (can cancel)
@@ -161,34 +157,27 @@ pi.on("session_start", async (_event, ctx) => {
 
 #### session_before_switch / session_switch
 
-Fired when switching sessions via `/resume`.
+Fired when starting a new session (`/new`) or switching sessions (`/resume`).
 
 ```typescript
 pi.on("session_before_switch", async (event, ctx) => {
-  // event.targetSessionFile - session we're switching to
-  return { cancel: true }; // Cancel the switch
+  // event.reason - "new" (starting fresh) or "resume" (switching to existing)
+  // event.targetSessionFile - session we're switching to (only for "resume")
+  
+  if (event.reason === "new") {
+    const ok = await ctx.ui.confirm("Clear?", "Delete all messages?");
+    if (!ok) return { cancel: true };
+  }
+  
+  return { cancel: true }; // Cancel the switch/new
 });
 
 pi.on("session_switch", async (event, ctx) => {
+  // event.reason - "new" or "resume"
   // event.previousSessionFile - session we came from
 });
 ```
 
-#### session_before_new / session_new
-
-Fired when starting a new session via `/new`.
-
-```typescript
-pi.on("session_before_new", async (_event, ctx) => {
-  const ok = await ctx.ui.confirm("Clear?", "Delete all messages?");
-  if (!ok) return { cancel: true };
-});
-
-pi.on("session_new", async (_event, ctx) => {
-  // New session started
-});
-```
-
 #### session_before_branch / session_branch
 
 Fired when branching via `/branch`.