resolve agent restoration (#1898)

[Rayatcoding]

Contribution path

• Linked issue or discussion: Agent Restoration #1898

Problem

Agent conversation state lives only in memory (McpClient._history). After dimos stop / process exit, the agent forgets prior turns. There is no way to resume a previous chat by session id.

Solution

• Persist _history to STATE_DIR/agent_sessions/<UUID>.json
• New dimos run assigns a fresh session UUID; restore only with --restore-session=<UUID>
• CLI helpers: dimos agent sessions / update-summary / delete
• Auto summary from first user message; optional user_summary for a custom display name

How to Test

Assumes a normal DimOS install (uv sync --extra all / see AGENTS.md) with dimos on PATH and OPENAI_API_KEY exported. Use two terminals if needed (one for dimos run, one for dimos agent-send).
Manual:

# Terminal 1 — start an agentic blueprint; note the printed session UUID
dimos run demo-skill
# Terminal 2 — send any chat message to the running agent
dimos agent-send "<your message>"
# Stop the run, then list saved sessions
dimos stop
dimos agent sessions
# expect: your UUID appears with a short summary and message count > 0
# Start again with the same UUID to restore history
dimos run demo-skill --restore-session=<UUID>
# Ask something that only makes sense if prior turns were restored
# (e.g. "What did I just ask you?")
dimos agent-send "<follow-up that depends on earlier context>"
# expect: agent continues from restored history, not a blank new session

Automated:
uv run pytest dimos/agents/mcp/test_session_store.py dimos/robot/cli/test_dimos.py::test_apply_agent_session_cli_assigns_new_id dimos/robot/cli/test_dimos.py::test_apply_agent_session_cli_restore dimos/robot/cli/test_dimos.py::test_apply_agent_session_cli_ignored_without_mcpclient dimos/robot/cli/test_dimos.py::test_apply_agent_session_cli_rejects_bad_id -q

## AI assistance

Cursor (assisted implementation). I reviewed and understand the changes.

## Checklist

- [x ] I have read and approved the [CLA](https://github.com/dimensionalOS/dimos/blob/main/CLA.md).