[Feat] BLS Define session state store interface (read / write / subscribe)

[pregnor]

Phase 1, interface definition

The state API and SSE endpoint read from the store; the A2A middleware writes and notifies subscribers.
Define the store’s surface so store implementation, state API, SSE, and middleware can be implemented in parallel without blocking each other.

• Specify: get state by session_id (optional topology_only), write/merge state for session, “push event” for session (payload conforms to session state progress schema), subscribe(session_id) returning a stream (or callback) of events for that session.
• Document in ADR or interface doc (docs/adr/* for example); no implementation in this ticket.
• Constraints: in-memory, keyed by session_id; no persistence.