fix(sandbox): round 7 align spawn evidence

packages/bash/bash-sandbox/README.md

@@ -17,7 +17,7 @@ Every command is confined by handing the provider the exact `['bash', '-c', comm
 Semantics:
 
 - **Denials are result facts.** A failed run whose stderr carries the selected backend's own denial dialect — the signatures the provider stamps on every wrap (EROFS text under bwrap, EACCES under Landlock, EPERM under Seatbelt) — is reported as `BashRunResult.sandbox.denied: true` (conservative classification, read from the collected stderr tail); every CONFINED run also carries the mode it executed under (`result.sandbox.mode`) and the provider's enforcement completeness (`result.sandbox.enforcement`: `full`, or `partial` on an older Landlock ABI).
-- **Runner failures are sandbox failures, never command failures.** Before a process starts, a rejection is attributed to the runner only when the caller-owned workdir is independently usable and the error carries Node spawn provenance for provider argv[0] plus an executable-class code. This covers missing, non-executable, and unloadable runners, including a script whose shebang interpreter is unavailable; an invalid or unusable workdir, resource failure, unrelated syscall, or unstructured rejection retains the local executor's command-start failure semantics. Foreground execution and a synchronous background start throw `SANDBOX_UNAVAILABLE` with the original spawn detail, while asynchronous background settlement stamps `runnerFailed: true` and `denied: false`. After a process starts, a rule's optional exit-code gate and a remaining fatal stderr line must both match after exact informational-line exclusions. A match outranks denial; foreground execution throws `SANDBOX_UNAVAILABLE` with the matched fatal line, while a settled background process stamps `process.sandbox.runnerFailed`, which the bash producer renders through generic `task_output`. Confined background handles retain their mode/enforcement facts and release per-process accounting in either path.
+- **Runner attribution is conservative.** Before a process starts, a rejection is attributed to the runner only when the caller-owned workdir is independently usable and Node reports `ENOENT` or `EACCES` with positive provenance for provider argv[0]. This covers a missing runner, a non-executable runner, or an executable script whose shebang interpreter is unavailable. A bare `syscall: 'spawn'` without an exact error path, any other code, an invalid or unusable workdir, a resource failure, an unrelated syscall, or an unstructured rejection retains the local executor's command-start failure semantics. Foreground execution throws `SANDBOX_UNAVAILABLE` with the original spawn detail, while asynchronous background settlement stamps `runnerFailed: true` and `denied: false`. If a `SubprocessService` synchronously throws the same provenanced `ENOENT`/`EACCES` shape, background start throws `SANDBOX_UNAVAILABLE`; other synchronous errors propagate unchanged. After a process starts, a rule's optional exit-code gate and a remaining fatal stderr line must both match after exact informational-line exclusions. A match outranks denial; foreground execution throws `SANDBOX_UNAVAILABLE` with the matched fatal line, while a settled background process stamps `process.sandbox.runnerFailed`, which the bash producer renders through generic `task_output`. Confined background handles retain their mode/enforcement facts and release per-process accounting in either path.
 - **Deployment fallback, per-call policy.** [`ctx.sandboxPolicy`](../../sandbox/sandbox-policy/) resolves a complete `SandboxExecutionPolicy` for every tool call: the calling session supplies its mode override and immutable cwd root, while deployment config supplies the fallbacks for agentless calls. An approved escalation changes only that policy's mode; its session root stays attached. `resolve()` carries the policy onto the spec, so overlapping commands from different projects run, classify, and report under their own roots and modes. The capability fact `ctx.bash.sandboxMode` reports the configured default so the tool layer advertises escalation only when this executor is mounted; the static bash tool description separately owns denial and escalation guidance.
 - **File effects only.** Network and process visibility are deliberately not restricted — the mode vocabulary does not pretend to cover what the backend does not enforce.
 - Process mechanics (spawn, process-group kills, output collection/spill, background handles, credential scrub) are inherited from [`dsh-bash-local`](../bash-local/); runner selection lives in [`dsh-sandbox-local`](../../sandbox/sandbox-local/).
@@ -72,7 +72,7 @@ Append-only; newly visible content follows the reusable request prefix and does
 
 #### What the model sees
 
-If no runner can enforce a confined mode, the foreground call propagates the [`SANDBOX_UNAVAILABLE` error owned by `dsh-sandbox`](../../sandbox/sandbox/README.md#confinement-error-indirectly). A runner-attributable spawn failure supplies the original spawn error as detail; a rejection without executable-class argv[0] evidence remains an ordinary command-start error. A settled runner failure supplies the matched fatal stderr line and preserves the original stderr collection.
+If no runner can enforce a confined mode, the foreground call propagates the [`SANDBOX_UNAVAILABLE` error owned by `dsh-sandbox`](../../sandbox/sandbox/README.md#confinement-error-indirectly). A runner-attributable spawn failure supplies the original spawn error as detail; a rejection without `ENOENT`/`EACCES` argv[0] evidence remains an ordinary command-start error. A settled runner failure supplies the matched fatal stderr line and preserves the original stderr collection. When present, the appended `Runner failure: <detail>` is the authoritative diagnosis; the preceding backend-install text is the generic `SANDBOX_UNAVAILABLE` prefix.
 
 #### Token effect
 
@@ -86,5 +86,5 @@ Append-only; newly visible content follows the reusable request prefix and does
 
 - **Confinement covers file effects only** — network access and process visibility are unchanged, so the modes are not a general-purpose security sandbox.
 - **Denials are inferred from failed-command stderr** — backend signatures make the inference portable, but a matching application error can be classified as a denial and a denial omitted from the retained tail can be missed.
-- **A background runner failure has no immediate error channel** — it is recorded on the settled process and surfaces when the caller reads the generic task with `task_output`.
+- **An asynchronously observed background runner failure has no immediate error channel** — it is recorded on the settled process and surfaces when the caller reads the generic task with `task_output`; a provenanced synchronous `SubprocessService` throw instead fails `start()` immediately.
 - **`danger-full-access` deliberately bypasses `ctx.sandbox`** — it is an explicit unconfined mode, not a wider sandbox profile.