[Bug] onboard_developer MCP prompt fails: messages[0] must be Message or str, got dict

[janepark1248]

Bug Report

Version: 2.3.3
Environment: macOS, Claude Code CLI

Description

Running the /code-review-graph:onboard_developer slash command in Claude Code fails with the following error:

McpError: MCP error 0: Error rendering prompt 'onboard_developer': messages[0] must be Message or str, got dict.
Use Message({'role': 'user', 'content': '...'}) to wrap the value.

Steps to Reproduce

1. Install code-review-graph 2.3.3
2. Open Claude Code with the MCP server configured
3. Run /code-review-graph:onboard_developer
4. Error is thrown immediately

Root Cause (suspected)

The onboard_developer prompt handler appears to return a raw dict instead of a Message object (or str). The MCP protocol requires prompt handlers to return a Message object or string, not a plain dict.

The error message itself suggests the fix:

# Current (broken)
return {'role': 'user', 'content': '...'}

# Expected fix
return Message({'role': 'user', 'content': '...'})

Impact

The onboard_developer slash command is completely unusable. Other MCP tools (e.g. semantic_search_nodes, get_architecture_overview, detect_changes) work fine via direct tool calls.