fix(pi): written ownership claim, and the successor channel waits out its predecessor

The wrapper's claim is a written supersession before the channel acts;
the extension awaits (bounded) the previous channel's exit before
spawning its replacement, so a predecessor cannot drain queued frames
into the new session's records after the seed.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: schickling

hooks/pi-channel.ts

@@ -111,7 +111,7 @@ export default function (pi: ExtensionAPI) {
   };
 
   /** Open a channel and resolve with the hello's restored context (empty if none, or on timeout). */
-  const open = (ctx: ExtensionContext): Promise<string> => {
+  const open = async (ctx: ExtensionContext): Promise<string> => {
     if (!bin || !catalog || !identity) return Promise.resolve("");
     if (typeof ctx.isIdle !== "function") {
       // Refuse rather than degrade. Without a positive idle proof this extension cannot choose
@@ -124,9 +124,24 @@ export default function (pi: ExtensionAPI) {
       );
       return Promise.resolve("");
     }
-    // Closes the channel opened by the PREVIOUS session, whichever extension instance opened it.
-    closeChild(state.child);
+    // Closes the channel opened by the PREVIOUS session, whichever extension instance opened
+    // it — and WAITS (bounded) for it to exit before the replacement spawns: the successor
+    // shares the seat's record, and a predecessor draining its queued frames after the new
+    // session's seed would land stale state into fresh records.
+    const previous = state.child;
+    closeChild(previous);
+    await awaitExit(previous, 2000);
 
+    const awaitExit = (child: childProcess.ChildProcess | undefined, ms: number) =>
+      new Promise<void>((resolve) => {
+        if (!child || child.exitCode !== null || child.signalCode !== null) return resolve();
+        const timer = setTimeout(resolve, ms);
+        timer.unref?.();
+        child.once("exit", () => {
+          clearTimeout(timer);
+          resolve();
+        });
+      });
     const channelEnv: NodeJS.ProcessEnv = { ...process.env };
     if (runtimeId) channelEnv[RUNTIME_ID] = runtimeId;
     if (session) channelEnv[SESSION] = session;

src/pi_session.rs

@@ -64,7 +64,9 @@ pub fn run(
     let executable =
         std::env::current_exe().context("resolving st2 executable for the pi channel")?;
     let session = harness_state::session_token();
-    let seq = harness_state::claim_seq(&agent_dir);
+    // The claim is written: it supersedes whatever the predecessor left — including a
+    // still-fresh live record — before the channel or terminal writer act under it.
+    let seq = harness_state::claim(&agent_dir, identity.clone(), "pi", &session)?;
     let mut env = channel_env(
         &executable,
         catalog_root,