docs: add Shell Spawn Hard-Stop Rule for agent workflows (#258)

Documents fail-closed behavior when the shell executor fails to spawn
(exit_code: -1, empty stdout/stderr): probe once, mark shell unavailable,
hard-stop after two consecutive spawn failures, and emit a recovery report
(restart session, kill hung background terminals, prefer MCP-native paths)
instead of retry-spiralling.

- skills/llm-project-workflow/SKILL.md: portable rule section
- docs/llm-workflow-runbooks.md: Gitea runbook section
- gitea_mcp_server.py: shell_spawn_hard_stop operator guide rule
- tests/test_shell_spawn_hard_stop_docs.py: doc-contract tests (5)
- tests/test_operator_guide.py: require the new guide key

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>

docs/llm-workflow-runbooks.md

@@ -285,6 +285,33 @@ explicit control-checkout repair.
 
 Portable wording: [`skills/llm-project-workflow/SKILL.md`](../skills/llm-project-workflow/SKILL.md).
 
+## Shell Spawn Hard-Stop Rule
+
+Symptom: a shell tool call returns `exit_code: -1` with empty stdout/stderr.
+That is an executor spawn failure, not a command failure — the command never
+ran, and retrying the identical call cannot succeed.
+
+Required behavior (fail closed, issue #258):
+
+1. **Probe once.** On the first spawn failure, run one trivial probe
+   (`echo ok` or `pwd`). If the probe also returns `exit_code: -1`, mark
+   shell unavailable for the session.
+2. **Hard-stop at two.** After two consecutive spawn failures, stop all
+   further shell tool use for the session; never retry the same failing
+   spawn. A hundred retries produce a hundred identical failures (session
+   `019f382e`: 100+ tool calls stalled on a trivial encode-and-commit task).
+3. **Emit a recovery report.** The report must direct the operator to:
+   - restart the session,
+   - kill hung background terminals (a hung test runner holding the
+     executor is a known contributor),
+   - prefer MCP-native paths for remaining mutations (for example
+     `gitea_commit_files` under `gitea.repo.commit`) instead of shell.
+4. **No improvised fallbacks.** Shell unavailability never authorizes
+   WebFetch/browser/manual-encoding workarounds (see #260). No shell means
+   stop-and-report.
+
+Doc-contract tests: `tests/test_shell_spawn_hard_stop_docs.py`.
+
 ## Branch worktree isolation
 
 All LLM implementation and review work happens in an isolated branch worktree