Critical fixes to salvaged PR #73020:
- Use SessionDB.append_message instead of raw INSERT INTO messages.
The original used wrong column names (session_key/created_at vs
session_id/timestamp) and bypassed FTS indexing, session metadata
updates, display_kind, and all other columns append_message handles.
- Use get_hermes_home() instead of hardcoded Path.home()/'.hermes'.
Profile-aware path resolution under HERMES_HOME override and active
profile isolation.
- Add 11 tests covering flush, recovery, serialisation, edge cases.
