refactor(shell): remove the unused background shell collect and waiter API

Nothing polls a background shell since shell_collect was removed: results
arrive through onExit and run_shell stop cancels. Drop collect,
releaseWaiters, the completed-run ring, and the collect wait cap, and move
the registry tests onto onExit.

Author: TheGreatAxios

docs/IMPLEMENTATION.md

@@ -123,7 +123,7 @@ src/
     permission-plugin.ts       Tiered operator approval (owns catastrophic shell deny)
   shell/
     run-shell-authz.ts         Shared run_shell deny policy (gate-enforced)
-    background-shell.ts        Background run_shell registry (start/collect/cancel/disposeAll)
+    background-shell.ts        Background run_shell registry (start/cancel/disposeAll)
     verify-plugin.ts           Write/edit verification (per-path lock)
     file-mutation-lock.ts      Serialize mutations per file for verify
     lsp-hint-plugin.ts         TS/JS LSP setup hint on unavailable server
@@ -208,7 +208,7 @@ Listings are list-free, dumps are dump-locked: a bounded `ls`/`tree` prints name
 
 #### Background shell mode
 
-`run_shell` accepts `background: true` (shell-guard plugin, after the permission chain — a denied command spawns nothing). The starting call resolves `cwd` as usual and applies a per-call `timeout` only when passed (no 120s default on background), skips the pwd probe, spawns a detached process group via the registry in `src/shell/background-shell.ts`, and returns `{shell_id, status: "running"}` immediately; the retained shell cwd is never mutated by a background run. Limits: 8 running, 8 completed entries (ring; evicted ids collect as not-found — truncated output is spilled to a `tool-output:///bg-shell-<id>` blob named in the completion message). Completion is process-exit, not stdio-close: a grandchild holding the pipe does not delay the completion message. On process exit the host delivers `buildShellBackgroundMessage(exit)` (exit status, timed-out marker, ~2KB output preview, spill URI) through the same continuation channel as compaction — wired in all three loop hosts (TUI, exec, sub-agent). `run_shell` with `stop: <shell_id>` kills the process group; it is ungated by design (cancel only kills the session's own child), and a finished run needs no poll because its result is delivered on its own. Timeout keeps its meaning: expiry kills the group and reports exit code 124 with `timed_out: true`. The tool watchdog exempts background starts because the process outlives the turn. A worker whose capability filter drops `run_shell` has no background registry wired — background starts fail closed. Parent abort, deadline, and `close_agent` release parked registry waiters and dispose live background groups; `interrupt_agent` only releases waiters so the session (and its children) stay resumable. Toolset dispose calls `disposeAll("session closed")` before the posix teardown, so `/clear`, interrupt, and reload kill every live background process group.
+`run_shell` accepts `background: true` (shell-guard plugin, after the permission chain — a denied command spawns nothing). The starting call resolves `cwd` as usual and applies a per-call `timeout` only when passed (no 120s default on background), skips the pwd probe, spawns a detached process group via the registry in `src/shell/background-shell.ts`, and returns `{shell_id, status: "running"}` immediately; the retained shell cwd is never mutated by a background run. Limits: 8 running, 8 completed entries (ring; evicted ids collect as not-found — truncated output is spilled to a `tool-output:///bg-shell-<id>` blob named in the completion message). Completion is process-exit, not stdio-close: a grandchild holding the pipe does not delay the completion message. On process exit the host delivers `buildShellBackgroundMessage(exit)` (exit status, timed-out marker, ~2KB output preview, spill URI) through the same continuation channel as compaction — wired in all three loop hosts (TUI, exec, sub-agent). `run_shell` with `stop: <shell_id>` kills the process group; it is ungated by design (cancel only kills the session's own child), and a finished run needs no poll because its result is delivered on its own. Timeout keeps its meaning: expiry kills the group and reports exit code 124 with `timed_out: true`. The tool watchdog exempts background starts because the process outlives the turn. A worker whose capability filter drops `run_shell` has no background registry wired — background starts fail closed. Parent abort, deadline, and `close_agent` dispose live background groups; `interrupt_agent` leaves them so the session (and its children) stay resumable. Toolset dispose calls `disposeAll("session closed")` before the posix teardown, so `/clear`, interrupt, and reload kill every live background process group.
 
 - **Alt+Enter** queues a follow-up (kind `"queue"`) delivered only on **session-idle** — parent-idle **and** no live fleet lanes (`run` goes idle). Session-idle Alt+Enter is a no-op. **Ctrl+C** stops the run.
 

src/plugins/shell-guard-plugin.test.ts

@@ -10,6 +10,7 @@ import { spawnSync, type ChildProcess } from "node:child_process";
 import { randomUUID } from "node:crypto";
 
 import { createBackgroundShellRegistry } from "../shell/background-shell.js";
+import { createExitRecorder } from "../shell/background-shell-test-harness.js";
 import { createShellOutputFeed } from "../session/shell-output-feed.js";
 
 import {
@@ -267,7 +268,10 @@ describe("background run_shell (shellGuardPlugin)", () => {
   }
 
   test("background:true returns a running handle immediately", async () => {
-    const registry = createBackgroundShellRegistry();
+    const recorder = createExitRecorder();
+    const registry = createBackgroundShellRegistry({
+      onExit: recorder.onExit,
+    });
     const start = Date.now();
     const result = await runWith(registry, {
       id: "bg1",
@@ -283,8 +287,7 @@ describe("background run_shell (shellGuardPlugin)", () => {
     expect(parsed.shell_id.length).toBeGreaterThan(0);
     // The START call resolved while the child was still sleeping.
     expect(Date.now() - start).toBeLessThan(400);
-    const snapshot = await registry.collect(parsed.shell_id, 5_000);
-    expect(snapshot.state).toBe("completed");
+    await recorder.waitFor(parsed.shell_id);
     registry.disposeAll("test done");
   });
 
@@ -334,7 +337,10 @@ describe("background run_shell (shellGuardPlugin)", () => {
   });
 
   test("background omitted timeout skips the foreground default", async () => {
-    const registry = createBackgroundShellRegistry();
+    const recorder = createExitRecorder();
+    const registry = createBackgroundShellRegistry({
+      onExit: recorder.onExit,
+    });
     const handler = defined(
       shellGuardPlugin(process.cwd(), { defaultMs: 90 }, undefined, {
         getBackgroundShellRegistry: () => registry,
@@ -353,12 +359,9 @@ describe("background run_shell (shellGuardPlugin)", () => {
     );
     expect(result.isError).toBeUndefined();
     const parsed = JSON.parse(String(result.content)) as { shell_id: string };
-    const snapshot = await registry.collect(parsed.shell_id, 5_000);
-    expect(snapshot.state).toBe("completed");
-    if (snapshot.state === "completed") {
-      expect(snapshot.exit.timedOut).toBe(false);
-      expect(snapshot.exit.output).toContain("survived");
-    }
+    const exit = await recorder.waitFor(parsed.shell_id);
+    expect(exit.timedOut).toBe(false);
+    expect(exit.output).toContain("survived");
     registry.disposeAll("test done");
   });
 

src/shell/background-shell-test-harness.ts

@@ -0,0 +1,25 @@
+import type { BackgroundShellExit } from "./background-shell.js";
+
+/** Collects `onExit` deliveries so tests can await one shell's completion. */
+export function createExitRecorder(): {
+  onExit: (exit: BackgroundShellExit) => void;
+  waitFor: (id: string, timeoutMs?: number) => Promise<BackgroundShellExit>;
+} {
+  const exits = new Map<string, BackgroundShellExit>();
+  return {
+    onExit: (exit) => {
+      exits.set(exit.id, exit);
+    },
+    waitFor: async (id, timeoutMs = 5_000) => {
+      const deadline = Date.now() + timeoutMs;
+      while (Date.now() < deadline) {
+        const exit = exits.get(id);
+        if (exit !== undefined) return exit;
+        await new Promise((resolve) => setTimeout(resolve, 20));
+      }
+      throw new Error(
+        `no exit delivered for shell ${id} within ${timeoutMs}ms`,
+      );
+    },
+  };
+}