Mark Merged Atomicity Task Brief and Ledger#

Contract#

• Design: docs/reference/design/runtime/session/merged-head-atomicity.html.
• Base: 0402ce3f.
• Make merged_heads read-union-write atomic under the existing per-session index lock.
• Keep persistence outside that lock and preserve order, trimming, idempotence, and callers.
• Do not add a transaction abstraction or modify other SessionStore operations.

Verification gate#

pytest -q tests/unit/test_mark_merged_atomicity.py tests/unit/test_archive_agent.py tests/unit/test_session_index_consistency.py
ruff check openprogram/store/session/session_store.py tests/unit/test_mark_merged_atomicity.py
python -m tools.docs_site.build
python -m tools.docs_site.checklinks
git diff --check

Ledger#