fix(session): add /resume CLI handler, session log truncation guard, reopen_session API (#3315)

Three improvements salvaged from PR #3225 by Mibayy:

1. Add /resume slash command handler in CLI process_command(). The
   command was registered in the commands registry but had no handler,
   so typing /resume produced 'Unknown command'. The handler resolves
   by title or session ID, ends the current session cleanly, loads
   conversation history from SQLite, re-opens the target session, and
   syncs the AIAgent instance. Follows the same pattern as new_session().

2. Add truncation guard in _save_session_log(). When resuming a session
   whose messages weren't fully written to SQLite, the agent starts with
   partial history and the first save would overwrite the full JSON log
   on disk. The guard reads the existing file and skips the write if it
   already has more messages than the current batch.

3. Add reopen_session() method to SessionDB. Proper API for clearing
   ended_at/end_reason instead of reaching into _conn directly.

Note: Bug 1 from the original PR (INSERT OR IGNORE + _session_db = None)
is already fixed on main — skipped as redundant.

Closes #3123.

run_agent.py

@@ -2055,6 +2055,23 @@ class AIAgent:
                     msg["content"] = self._clean_session_content(msg["content"])
                 cleaned.append(msg)
 
+            # Guard: never overwrite a larger session log with fewer messages.
+            # This protects against data loss when --resume loads a session whose
+            # messages weren't fully written to SQLite — the resumed agent starts
+            # with partial history and would otherwise clobber the full JSON log.
+            if self.session_log_file.exists():
+                try:
+                    existing = json.loads(self.session_log_file.read_text(encoding="utf-8"))
+                    existing_count = existing.get("message_count", len(existing.get("messages", [])))
+                    if existing_count > len(cleaned):
+                        logging.debug(
+                            "Skipping session log overwrite: existing has %d messages, current has %d",
+                            existing_count, len(cleaned),
+                        )
+                        return
+                except Exception:
+                    pass  # corrupted existing file — allow the overwrite
+
             entry = {
                 "session_id": self.session_id,
                 "model": self.model,