fix: persist ACP sessions to SessionDB so they survive process restarts

* fix: persist ACP sessions to disk so they survive process restarts

The ACP adapter stored sessions entirely in-memory. When the editor
restarted the ACP subprocess (idle timeout, crash, system sleep/wake,
editor restart), all sessions were lost. The editor's load_session /
resume_session calls would fail to find the session, forcing a new
empty session and losing all conversation history.

Changes:
- SessionManager now persists each session as a JSON file under
  ~/.hermes/acp_sessions/<session_id>.json
- get_session() transparently restores from disk when not in memory
- update_cwd(), fork_session(), list_sessions() all check disk
- server.py calls save_session() after prompt completion, /reset,
  /compact, and model switches
- cleanup() and remove_session() delete disk files too
- Sessions have a 7-day TTL; expired sessions are pruned on startup
- Atomic writes via tempfile + os.replace to prevent corruption
- 11 new tests covering persistence, disk restoration, and TTL expiry

* refactor: use SessionDB instead of JSON files for ACP session persistence

Replace the standalone JSON file persistence layer with SessionDB
(~/.hermes/state.db) integration. ACP sessions now:
- Share the same DB as CLI and gateway sessions
- Are searchable via session_search (FTS5)
- Get token tracking, cost tracking, and session titles for free
- Follow existing session pruning policies

Key changes:
- _get_db() lazily creates a SessionDB, resolving HERMES_HOME
  dynamically (not at import time) for test compatibility
- _persist() creates session record + replaces messages in DB
- _restore() loads from DB with source='acp' filter
- cwd stored in model_config JSON field (no schema migration)
- Model values coerced to str to handle mock agents in tests
- Removed: json files, sessions_dir, ttl_days, _expire logic
- Tests updated: DB-backed persistence, FTS search, tool_call
  round-tripping, source filtering

---------

Co-authored-by: Test <test@test.com>

acp_adapter/server.py

@@ -304,6 +304,8 @@ class HermesACPAgent(acp.Agent):
 
         if result.get("messages"):
             state.history = result["messages"]
+            # Persist updated history so sessions survive process restarts.
+            self.session_manager.save_session(session_id)
 
         final_response = result.get("final_response", "")
         if final_response and conn:
@@ -400,6 +402,7 @@ class HermesACPAgent(acp.Agent):
             cwd=state.cwd,
             model=new_model,
         )
+        self.session_manager.save_session(state.session_id)
         provider_label = target_provider or getattr(state.agent, "provider", "auto")
         logger.info("Session %s: model switched to %s", state.session_id, new_model)
         return f"Model switched to: {new_model}\nProvider: {provider_label}"
@@ -444,6 +447,7 @@ class HermesACPAgent(acp.Agent):
 
     def _cmd_reset(self, args: str, state: SessionState) -> str:
         state.history.clear()
+        self.session_manager.save_session(state.session_id)
         return "Conversation history cleared."
 
     def _cmd_compact(self, args: str, state: SessionState) -> str:
@@ -453,6 +457,7 @@ class HermesACPAgent(acp.Agent):
             agent = state.agent
             if hasattr(agent, "compress_context"):
                 agent.compress_context(state.history)
+                self.session_manager.save_session(state.session_id)
                 return f"Context compressed. Messages: {len(state.history)}"
             return "Context compression not available for this agent."
         except Exception as e:
@@ -475,5 +480,6 @@ class HermesACPAgent(acp.Agent):
                 cwd=state.cwd,
                 model=model_id,
             )
+            self.session_manager.save_session(session_id)
             logger.info("Session %s: model switched to %s", session_id, model_id)
         return None