[codex] Persist multi-agent version metadata

[aibrahim-oai]

Why

The runtime pinning change needs a durable place to store the selected multi-agent system before any behavior changes are introduced.

What changed

• Add the shared MultiAgentVersion enum with v1 and v2.
• Add optional version fields to rollout metadata, thread-store metadata, and SQLite.
• Read and write the value through rollout, thread store, and state DB paths.
• Read the latest matching rollout metadata value while preserving the canonical thread identity.
• Add storage round-trip and tolerant protocol coverage. Unknown values collapse to omission.

This PR is structural only. It does not resolve flags or change runtime gates.

Verification

Ran just fmt. Validation is running in online CI.

Stack

1. This PR: [codex] Persist multi-agent version metadata #25153
2. Runtime pinning: [codex] Pin multi-agent system per thread #25168
3. ModelInfo overlay: [codex] Add model multi-agent system overlay #25155

Supersedes #25032.

[aibrahim-oai]

This change is part of the following stack:

• [codex] Persist multi-agent version metadata #25153 ◀

_{Change managed by git-spice.}

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Preserve stored multi-agent version on resume

When resuming an agent from a thread store whose selector lives in StoredThread.multi_agent_version rather than in a SessionMeta rollout item, passing None here makes Codex::spawn resolve v1/v2 from the current feature flags instead. This already applies to the in-memory/pathless store shape, where creation metadata can carry the selector while the replay history does not, so a resumed thread can switch multi-agent systems despite the stored metadata that resume_single_agent_from_rollout just read.

Useful? React with 👍 / 👎.

[chatgpt-codex-connector]

Skip unpinned metadata entries when resolving version

When a legacy rollout is first pinned, it appends a later SessionMeta with multi_agent_version, but existing compatibility updates such as memory-mode or git updates append another SessionMeta cloned from the original head record, where this field is still omitted. Because this reverse scan stops at that newest SessionMeta even when its value is None, the next resume/fork treats the thread as unpinned and can switch back to whatever the current feature flags say; keep searching for an actual version or ensure metadata-only SessionMeta updates preserve the pinned selector.

Useful? React with 👍 / 👎.