Manual meta checkpoint: /checkpoint command, save-on-quit, and a save_checkpoint tool

[ziatdinovmax]

Summary

The meta (explore) session previously saved its state only automatically — every 10 messages, or when a chat error occurred. Quitting at the wrong moment could silently lose up to 9 messages of session state, including the record of recent delegations, and there was no way to ask for a save explicitly.

This PR adds manual checkpointing at the meta level, on all surfaces:

• /checkpoint CLI command — saves the session state immediately and prints the checkpoint path (listed in /help).
• Checkpoint on exit — /quit, /exit, and Ctrl-C/EOF now save a checkpoint before leaving, so a resumed session always has the full delegation record.
• "Save a checkpoint" from chat, including the UI — the meta now has a save_checkpoint tool (the planning and analysis specialists already had one), so asking the meta to save the session works in the UI chat as well as the CLI.

A failed explicit save now reports an error instead of quietly logging a warning, so a manual save can't silently do nothing.

---

Technical details

• MetaOrchestratorAgent.save_checkpoint() (meta_orchestrator.py): public on-demand save. Wraps _auto_checkpoint() (which now returns a bool), flushes chat history via _save_history(), resets last_checkpoint_message_count so the 10-message auto-save timer restarts, returns the checkpoint path, and raises RuntimeError on write failure.
• save_checkpoint meta tool (meta_orchestrator_tools.py): registered alongside get_delegation_history, mirroring the children's tool shape; returns {status, checkpoint_path, delegations_saved} JSON. This is what makes the feature reachable from the UI — the UI chat input passes text straight to the agent with no slash-command interception, so tool-mediated saving is the UI path.
• CLI (cli/meta.py): _save_checkpoint() helper (reports failure, never raises into the loop), /checkpoint command, save on the QUIT branch and on the EOFError/KeyboardInterrupt branch of the main loop.
• Checkpoint content is unchanged (meta mode, message count, instantiated children, delegation ledger); children continue to checkpoint themselves in their own sub-directories.

Testing: offline smoke tests — happy path writes correct state and resets the timer; unwritable path raises RuntimeError; tool wrapper returns clean success/error JSON; /checkpoint handles success and failure without breaking the CLI loop; case-insensitive command parsing. Existing offline meta suites pass (21/21: test_meta_simulation_delegation, test_meta_fanout_robustness, test_file_prep_metadata_only).

Known limits: no live end-to-end run of the UI chat → save_checkpoint tool → resume cycle yet; the UI still has no dedicated save button (chat-mediated only).