Add pluggable SessionStore interface with FSJSONL + Null backends

Introduce a neutral persistence seam (session_store.zig) following the
{ptr, vtable} shape of the other libpanto seams. The interface traffics in
DiskMessage so non-JSONL backends (e.g. Postgres) can implement it.

- session_store.zig: SessionStore vtable (appendMessages, loadConversation,
  sessionId, activeModel), LoadedSession{conversation, dangling_user},
  re-exported disk types, and an FSJSONLStore alias.
- session_manager.zig: add store() wrapper + loadConversation() with
  dangling trailing-user detection (excluded from the rebuilt conversation,
  returned as dangling_user).
- null_store.zig: no-op backend, stateless singleton.
- root.zig: export session_store + null_store.

Phase 1+2 of docs/pluggable-session-store.md. Behavior-preserving; CLI not
yet rewired.

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