Assistant: Initial experimental git integration

[georgestagg]

This PR adds initial experimental git integration with Positron Assistant.

• Adds a button to the commit message input box to generate a commit message using LLM (addresses Enhancement: make it possible for LLMs to craft commit messages #8072)
• Adds a tool call and chat context item #changes to provide current git changes as context to a chat session (partially addresses (Assistant: I wish I could expose current diff or a specific commit as context #8207)

Both features are hidden behind an experimental config switch for the moment.

QA Notes

1. Enable the experimental config switch:

    "positron.assistant.gitIntegration.enable": true,

2. Open a workspace containing an active git repo. Make some changes for the LLM to see. Optionally stage some of them.

3. Ask the LLM about the changes (optionally hitting # and adding the new context item #changes). It should be able to see both your staged and unstaged changes.

4. In the Git pane, hit the new sparkle button to generate a git commit message. If there are staged changes, only they should be considered. Otherwise, all changes and new untracked files should be considered.

[github-actions]

E2E Tests 🚀
This PR will run tests tagged with: @:critical

^{readme  valid tags}

[seeM]

I like it!

#changes works as expected in the chat pane ✅

I can generate the commit message, and cancel generation ✅

Screen.Recording.2025-06-25.at.14.24.57.mov

And neither of the above appear when the feature flag is disabled.