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/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 */