Add read_file skill, session persistence, and update script

- New read_file skill: paginated file reading with line ranges,
  path restricted to /workspace, binary detection, directory listing
- Session persistence via SQLite + FTS5: conversation history survives
  agent restarts, last N messages restored into deque on boot,
  auto-prune to 1000 messages
- Update truncation hint to reference read_file instead of run_command
- New scripts/update.sh for patching rootfs + rebuilding snapshot

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

agent/skills.py

@@ -166,7 +166,7 @@ def execute_skill(script_path, args, workspace, config):
             with open(filepath, "w") as f:
                 f.write(output)
             preview = output[:1500]
-            return f"{preview}\n\n[output truncated — full result ({len(output)} chars) saved to {filepath}. Use run_command to read it: cat {filepath}]"
+            return f"{preview}\n\n[output truncated — full result ({len(output)} chars) saved to {filepath}. Use read_file to view it.]"
 
         return output
     except subprocess.TimeoutExpired: