MM-15: Add rebuildable per-session context manifest

Parent

#771

Checkpoint

MC-4A Session continuity

What to build

Maintain a versioned per-session metadata projection for source ids, snippets, token estimates, protection, relevance, and archive references that can be rebuilt from the transcript.

Acceptance criteria

• Incremental updates are idempotent.
• Corrupt/missing manifests rebuild from RuntimeEvent history.
• Manifest deletion never loses transcript facts.
• Protected source rules are deterministic.
• Secrets and hidden reasoning are excluded.

Required tests

• Append/update
• schema migration
• corruption/rebuild
• crash mid-write
• protected-rule matrix

Benchmark

Long-session manifest growth and assembly-latency microbenchmark.

Blocked by

• [MM-14] Contract legacy provider-visible rewrite paths #793 (MM-14)

Rollout and rollback

Delete/rebuild the projection and assemble from transcript through the compatibility strategy.