fix(e2b): harden cancellation and teardown boundaries

packages/e2b/subprocess-e2b/README.md

@@ -10,8 +10,9 @@ E2B implementation of the [`@deepseek-ai/dsh-subprocess`](../../subprocess/subpr
 - **Execution-world coordinates** — `cwd` and private `runtimeRoot` come from the shared owner; executable lookup verifies absolute paths or resolves a bare name against the sandbox PATH plus explicit overrides.
 - **Linux process groups** — a quoted wrapper starts each argv under `exec setsid --wait` and records its actual process-group id plus private status files beneath `ctx.e2b.runtimeRoot/processes`. The handle waits for that file instead of treating the SDK command PID as its published identity. Termination signals the negative recorded id with `SIGTERM`, waits the caller's `graceMs`, then escalates to `SIGKILL` and the SDK kill fallback; TERM delivery or probe failures also force that escalation. Process-table probes treat groups containing only zombie or dead entries as quiescent. A failed transaction is observable through `waitForExit()` and may be retried, while any proven quiescence permanently fences later termination against PID reuse. Before publication, cancellation uses the provisional SDK handle; if publication fails, rollback kills and verifies the provisional group before startup rejects. After publication, a monitoring failure also rolls back the group before rejecting. Service disposal rejects new starts, terminates and joins every retained process group, then awaits SDK settlement and private cleanup before the sandbox owner disposes.
 - **Environment boundary** — the wrapper starts from the sandbox command environment, removes ambient `DSH_*` and credential-shaped (`*KEY*`, `*SECRET*`, `*TOKEN*`) names, then restores every valid `spec.env` entry as an explicit caller opt-in; empty names, `=`, and NUL framing violations reject before launch. Host ambient variables never enter the sandbox implicitly. Private environment files are removed after consumption, and failed command or terminal setup removes its private state before rejecting.
-- **Stdio projection** — the remote wrapper branches raw bytes into optional bounded spill files, frames each live chunk as newline-delimited base64 ASCII, and the host incrementally restores bytes across arbitrary SDK callback boundaries. Pipe mode writes those bytes to host Node streams; inherit mode writes them to the harness process streams; collect mode retains a bounded host tail with offset reads. The wrapper publishes the direct command status before waiting for inherited writers. For collect or inherit output, the adapter disconnects an incomplete SDK stream after `graceMs`, withholds its partial spill, and returns that status while retaining the remote group for `waitForExit()` and termination; raw pipes instead await lossless transport completion and preserve backpressure. Batch and streaming stdin use the SDK handle.
-- **Terminal sessions** — `spawnTerminal()` uses E2B's byte PTY API, installs the exact argv and scrubbed environment through private mode-`0600` files, reports the foreground process group, sends real signals, and tears down every live group in the remote terminal session before settlement; zombie-only groups are already quiescent. A private random output boundary discards the E2B bootstrap shell's prompt and echoed runner command while preserving every requested-process byte, including its first prompt. Setup and teardown own the private state transaction, abort pending setup during service disposal, fence publication, and retain an unproven setup cleanup for disposal retry. Prompt detection, scrollback, readiness, and owner policy remain in `dsh-pty-local`.
+- **Stdio projection** — the remote wrapper branches raw bytes into optional bounded spill files, frames each live chunk as newline-delimited base64 ASCII, and the host incrementally restores bytes across arbitrary SDK callback boundaries. Pipe mode writes those bytes to host Node streams; inherit mode writes them to the harness process streams; collect mode retains a bounded host tail with offset reads. The wrapper publishes the direct command status before waiting for inherited writers. For collect or inherit output, the adapter disconnects an incomplete SDK stream after `graceMs`, withholds its partial spill, and returns that status while retaining the remote group for `waitForExit()` and termination. Natural raw-pipe completion instead awaits lossless transport and preserves backpressure; explicit termination destroys the host pipes and releases blocked output before remote cleanup. Batch and streaming stdin use the SDK handle.
+- **Terminal sessions** — `spawnTerminal()` uses E2B's byte PTY API, installs the exact argv and scrubbed environment through private mode-`0600` files, reports the foreground process group, sends real signals, and tears down every live group in the remote terminal session before settlement; zombie-only groups are already quiescent. A private random output boundary discards the E2B bootstrap shell's prompt and echoed runner command while preserving every requested-process byte, including its first prompt. PTY allocation is awaited through handle publication before cancellation is observed, so owned rollback can clean the published handle. Setup and teardown own the private state transaction, abort pending setup during service disposal, fence publication, and retain an unproven setup cleanup for disposal retry. Prompt detection, scrollback, readiness, and owner policy remain in `dsh-pty-local`.
+- **Sandbox disappearance** — `SandboxNotFoundError` during process or terminal liveness, termination, rollback, or disconnect proves the remote execution world cannot retain work, so cleanup treats it as quiescent; unrelated failures remain observable.
 
 The base E2B image supplies the runtime and Bash/GNU utilities this adapter invokes: `node`, `bash`, `setsid`, `ps`, `awk`, `tr`, `env`, `chmod`, `tee`, `head`, `rm`, and `kill`. A custom template must retain compatible commands and E2B PTY support.