chore: snapshot working tree - pty_exited notifications + in-flight inference WIP

feat(booterm): structured pty_exited WS notifications. Plan-validated, impl-validated, code-reviewed green (contracts build clean, contracts test 29/29, booterm + web typecheck clean).

wip: in-progress inference/provider refactor (agents.ts, provider.ts, new llama-providers.ts, removed llama-args-validator), plus arena, dispatcher, compaction, schema changes.

openspec: pty-exit-notifications complete; x-agent-flags planned (not yet implemented).

apps/booterm/src/pty/manager.ts

@@ -182,6 +182,7 @@ export async function sweepExpired(
         ? 'idle timeout'
         : 'absolute timeout';
     log.info({ paneId: meta.paneId, reason }, 'sweeping expired PTY session');
+    meta.timedOut = true;
     const sessionName = tmuxSessionName(meta.paneId);
     try {
       const ok = await killSession(tmuxConfPath, sessionName);
@@ -191,7 +192,6 @@ export async function sweepExpired(
     } catch (err) {
       log.warn({ paneId: meta.paneId, err }, 'killSession threw during sweep');
     }
-    registry.unregister(meta.paneId);
     killed.push(meta.paneId);
   }
   return killed;

apps/booterm/src/pty/registry.ts

@@ -10,6 +10,7 @@ export interface SessionMeta {
   timeoutSeconds?: number;
   idleExpiresAt?: Date;
   absoluteExpiresAt?: Date;
+  timedOut?: boolean;
 }
 
 const sessions = new Map<string, SessionMeta>();
@@ -115,6 +116,18 @@ export interface SearchMatch {
 
 const ringBuffers = new Map<string, string[]>();
 
+/**
+ * Return the last N non-empty lines from the ring buffer for a pane.
+ * ANSI escape sequences are preserved (xterm handles them).
+ * Partial lines from mid-stream exit are included as-is.
+ */
+export function getLastLines(paneId: string, n: number): string[] {
+  const buf = ringBuffers.get(paneId);
+  if (!buf || buf.length === 0) return [];
+  const nonEmpty = buf.filter(l => l.trim().length > 0);
+  return nonEmpty.slice(-n);
+}
+
 /**
  * Append raw PTY data to the ring buffer for a given pane.
  * Splits incoming data on newlines and pushes each line into the buffer,

apps/booterm/src/ws/attach.ts

@@ -9,7 +9,7 @@ import {
 } from '../pty/manager.js';
 import { attachPty } from '../pty/pty.js';
 import { getUser } from '../auth.js';
-import { register, unregister, appendOutput, touchActivity, consumePendingMetadata } from '../pty/registry.js';
+import { register, unregister, appendOutput, touchActivity, consumePendingMetadata, get as getRegistry, getLastLines } from '../pty/registry.js';
 
 export function registerWsAttachRoute(
   app: FastifyInstance,
@@ -168,9 +168,22 @@ export function registerWsAttachRoute(
       });
 
       handle.onExit(({ exitCode }) => {
+        const meta = getRegistry(pid);
+        const lastLines = getLastLines(pid, 5);
+        const frame = {
+          type: 'pty_exited' as const,
+          session_id: sid,
+          pane_id: pid,
+          exit_code: exitCode,
+          last_lines: lastLines,
+          session_title: meta?.title ?? null,
+          session_description: meta?.description ?? null,
+          parent_agent: meta?.parentAgent ?? null,
+          timed_out: meta?.timedOut ?? false,
+        };
         try {
           if (socket.readyState === socket.OPEN) {
-            socket.send(JSON.stringify({ type: 'exit', code: exitCode }));
+            socket.send(JSON.stringify(frame));
           }
         } catch {
           /* ignore */