MCP Update tool

[ChrisPenner]

Overview

• Allows agents to call "update" via the MCP server
• This requires the user to explicitly mark a branch as editable by the mcp by passing the --mcp-editable-branches=<project/branch regex> flag in their MCP config. We can continue to explore other options for this.
• I haven't yet had time to fully explore how this flow works in cases where update requires user interaction; I'd love to hear reports from dogfooding before we merge.

Implementation notes

• Adds the new global --mcp-editable-branches=<project/branch regex> flag
• Adds a new update tool
  • accepts literal code or a filepath to a scratch file.
• Updates the error handling for the CLI to MCP runner so we can short-circuit if code doesn't typecheck.

Test coverage

Added an API transcript test
Could use some dogfooding and testing in the wile before merging/releasing

Loose ends

May want to explore other methods of allowing mcp edits on certain branches, editing your config file and restarting your agent every time is a pain.

[aryairani]

Let's do the thing where we mark it as destructive but otherwise don't block.
And maybe later we do the thing where you manually whitelist branches with ucm commands.
As long as we don't have delete brach we should be okay with undo and change comments, though the agent can also just delete files from the disk if you let it...

[ChrisPenner]

Even if it did delete a branch I think you could still get it back with the global reflog, but obviously not ideal.

Okay sounds like a plan 👍🏼

[ChrisPenner]

@aryairani Okay I removed the branch-protection stuff.