feat: serialize autonomous release runs (#335)

docs/release-engine-spec.md

@@ -44,6 +44,19 @@ repeating or advancing delivery.
 4. Otherwise sort by priority label (`P0`, `P1`, `P2`, unlabeled) then issue number.
 5. Select exactly one issue per invocation.
 
+## Single-active-run lease
+
+- Before issue discovery, every invocation acquires a non-blocking advisory lock
+  at `.release-engine/run.lock` under the canonical repository path.
+- The lease is held through the final checkpoint and pull-request request. A
+  contended invocation fails immediately with `release run already active`
+  before contacting Gitea or running Git, agent, or test commands.
+- The lock file records bounded owner PID and acquisition-time diagnostics.
+  These are informational only: kernel lock ownership is authoritative, so a
+  killed or crashed process releases the lease automatically and stale metadata
+  cannot block the next run.
+- Different canonical repository paths use independent leases.
+
 ## Claiming
 
 - PATCH the issue with the configured assignee.
@@ -78,6 +91,10 @@ repeating or advancing delivery.
 
 - `--dry-run` performs discovery and planning only: no claim, git mutation, agent command, push, or PR.
 - State is written atomically after each successful transition.
+- Each checkpoint write uses a writer-unique temporary file, flushes file data,
+  atomically replaces the checkpoint, and flushes its parent directory. A
+  failed or concurrent writer therefore cannot collide on a shared temp path or
+  expose partial JSON.
 - Resumable state carries repository and commit identity; successful stages are
   skipped only after local (and, after push, remote) Git verification.
 - One invocation handles at most one issue.
@@ -101,3 +118,5 @@ repeating or advancing delivery.
 13. Dirty or identity-mismatched checkpoints stop before push or PR creation.
 14. Parent credentials are absent from coding-agent, test, and Git subprocesses while explicit agent variables and issue context reach only the coding stage.
 15. Reserved or malformed agent environment allowlist entries abort before claim.
+16. An overlapping invocation fails before Gitea discovery or local command execution and reports bounded owner diagnostics.
+17. Concurrent checkpoint-save stress completes without temp-file collisions or partial state.