Closes#11360.
## Summary
- Cap combined stdout/stderr with `ExecOptions.maxBuffer`, defaulting to
the smaller of 256 MiB and the runtime string limit. Invalid explicit
limits reject before spawning.
- Terminate oversized commands and return `truncated: true`, `killed:
true`, and a nonzero exit code. Close output pipes so writing
descendants cannot keep the call pending, and discard incomplete UTF-8
suffixes rather than adding replacement characters.
- Fix SIGKILL escalation for children that ignore SIGTERM, and simplify
completion to `await` with one cleanup block and result.
- Document the API and add regressions for byte limits, shell
descendants, UTF-8 chunk boundaries, and 32-bit runtime limits.
`ExecResult.truncated` is required. Existing extensions reading
execution results continue working; typed mocks or wrappers constructing
results must add the field.
## Verification
- `npm run build` and the post-rebase `npm run build:offline`
- `npm run check`
- 39 focused execution and extension tests
- `./test.sh` passes after building the env daemon. Earlier runs hit
remote-cleanup and reftable-watcher timeouts; both passed in focused
reruns and the final full run.
