fix: snapshot persistence rewrites transcripts without updating user-turn/activity metadata

[sam-saffron-jarvis]

What changed

• Updated internal/session.SQLiteStore.ReplaceMessages to recompute and persist session metadata in the same transaction as the transcript rewrite.
• The replace path now refreshes:
  • user_turns
  • last_user_message_at
  • last_message_at
  • existing compaction-boundary clearing / updated_at
• Added a regression test covering both failure modes:
  • initial ReplaceMessages must populate the metadata from the persisted transcript
  • rerunning ReplaceMessages with an unchanged prefix must preserve the original persisted activity timestamps instead of deriving them from freshly rebuilt in-memory snapshot timestamps

Why this is high-value

persistSnapshot is used for durable pre-stream saves, replace-history runs, and reconciliation after persistence errors. Before this change, it could successfully rewrite the transcript rows while leaving session metadata stale because it relied on store.Update(...), and Update intentionally does not write turn counters. That produced real sessions whose transcript existed but whose user-turn count and activity timestamps stayed at 0/NULL/old values.

This directly affects sidebar ordering and exported session stats, especially when a run fails before later append-only callbacks can repair the metadata.

Validation

• gofmt -w internal/session/sqlite.go internal/session/sqlite_test.go
• go build ./...
• go test ./...
• Added regression test: TestSQLiteStoreReplaceMessagesRecomputesActivityMetadata