pre-push(17.5): say when the engine cannot finish, and offer it in the background

--engine inline|background|off (hook: OUSAST_PUSH_ENGINE). background returns
after the quick tier and starts one detached explicit replay of the same base
and head (push/background.py), one at a time per artifact directory; its
artifact lands under <artifact dir>/engine/<repo key>/<head>.json and the next
run prints "Engine result for <head> from the previous push" once. A deadline
miss says so in one line naming the deadline and the background option. No
engine speed work. Requirement 9.5.

Co-Authored-By: Claude Opus 5.5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01EmM1j46qgKMh3zqD1QMwEo

.kiro/specs/pre-push-safety-net/tasks.md

@@ -648,10 +648,11 @@ Preserve all completed task evidence below.
   - Evidence: unit tests on the comparison and admission; a live engine run if the host allows one (free memory >= 3 GB).
   - _Done 2026-10-02:_ `ChangeContext.added_paths` records Git status-A paths (rename targets excluded). In `_compare_evidence`, after every existing context/head/deadline check, an operation in an added file is `new`/`file_added_in_head` when the claim is entailed, undischarged and has change evidence, and no base answer holds the same operation (else `operation_moved_within_change`); an unpaired add/delete still blocks as `change_context_unresolved`. Admission accepts `(new, file_added_in_head)`; with the empty registry such a candidate's only veto is `capability_unavailable`, and the terminal now shows it under "Engine: N finding(s) this push introduced. Advisory ... not an alert." Any comparison, evidence or location veto keeps a candidate out of that line. Tests: `tests/test_push_new_file.py` (9). Live engine evidence depends on host memory (17.7). Gates: 2007 passed, 42 skipped; ruff, format, mypy clean.
   - _Requirements: 9.4, 3.4_
-- [ ] 17.5 Say when the engine cannot finish, and offer it in the background
+- [x] 17.5 Say when the engine cannot finish, and offer it in the background
   - One line when the engine did not finish (deadline, engine not installed). `--engine background` starts the engine detached after the quick tier, writes its result under the artifact directory and, on the next run, prints "engine result for <head> from the previous push". One background engine at a time; `--engine off` skips it with a stated line. No speed work.
   - Files: `src/openultrasast/push/background.py` (new), `src/openultrasast/push/runner.py`, `src/openultrasast/cli.py`, `ops/pre-push`. Tests: `tests/test_push_background.py` (new).
   - Evidence: a hook push with `OUSAST_PUSH_ENGINE=background` followed by a second push showing the previous result.
+  - _Done 2026-10-02:_ `--engine inline|background|off` (hook: `OUSAST_PUSH_ENGINE`) and `--background-deadline` (default 900 s). `off` and `background` run the quick tier and record `engine_off`/`engine_background` with a plain skip line; the deadline line names the deadline and the background option. New `push/background.py` starts one detached explicit replay (`start_new_session`, stdin closed, output to a log) per artifact directory and repository (lock file with a live-pid check: a second start reports busy), writes its artifact under `<artifact dir>/engine/<repo key>/<head>.json`, and the next run prints "Engine result for <head> from the previous push: ..." once, then marks it shown. Not usable through the Docker wrapper (the container exits with the hook); documented in 17.6. Tests: `tests/test_push_background.py` (4, one launching a real detached run). Gates: 2011 passed, 42 skipped; ruff, format, mypy clean.
   - _Requirements: 9.5, 4.1, 4.2_
 - [ ] 17.6 Install, Docker and documentation for a first-time user
   - `ousast pre-push install|uninstall [--force] [--docker] [REPO]` ships the hook as package data, honours `core.hooksPath`, refuses an existing hook unless `--force` (which chains it as `pre-push.before-ousast`), and works from a pip install. `ops/ousast-docker` is a hook-callable script (not a shell function) that passes stdin and the artifact path through. The `openultrasast:dev` image is rebuilt only if `df -h /` shows at least 4 GB free; otherwise the rebuild command is documented and the record says it was not rebuilt. `docs/scanning.md` gains a first-run guide, a glossary of skip reasons, a GitHub Actions recipe and full `--help` text; README tiers 1 and 2 updated, pulling `ghcr.io/norandom/openultrasast:2.0.1` where the wrapper allows.