Refactor To-Do tools as internal MCP extension

[tlongwell-block]

THIS IS A ROUGH DRAFT TO SHOW A POC

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This PR refactors the TODO task management tools from hardcoded platform tools to an internal MCP extension, bringing them in line with our extension architecture.

What changed

• To-Do tools now register as a proper MCP extension when a session is available
• Created TodoMcpClient that implements the standard McpClientTrait interface
• Instructions moved from system prompt to extension metadata where they belong

Why this is better

• Consistency: TODO tools follow the same patterns as all other extensions
• Cleaner architecture: Tool logic is encapsulated, not scattered through the agent
• Better testability: The MCP client can be tested in isolation
• Smaller system prompt: Instructions come from the extension, reducing prompt overhead

Implementation notes

The tools continue to work exactly as before from the user's perspective. Session storage integration is preserved through a TodoStorage trait abstraction. The change touches minimal existing code - mostly just removing the hardcoded handling and adding the registration logic.

[michaelneale]

am a little confused - this is in process but looks like an MCP (with a faux-client?)

[tlongwell-block]

am a little confused - this is in process but looks like an MCP (with a faux-client?)

Yes, that's exactly what this is per #4454 (comment)

[tlongwell-block]

Closing to declutter open PRs since this might take a while to think through