Function write_session_state 

pub fn write_session_state(
    filestore: &FileStore,
    definition_id: &str,
    content: &[u8],
) -> Result<(), String>

Write content to output.state.json in agent:<defId>:current. Idempotent — creates the file if missing, overwrites otherwise.

The per-channel write is primary (preserving all existing same-channel behaviour); the snapshot overlay is additionally mirrored into the GLOBAL transcript store (when installed) so a cross-channel open — and the migrated-agent backfill — find a coherent zone (overlay + output together). Mirroring is best-effort: a global-store failure is logged, never propagated. See docs/analysis/ANALYSIS_CROSS_CHANNEL_CONVERSATION_HISTORY_2026_06_14.md.