feat: add configurable base-branch review mode

[werkamsus]

Summary

This adds a branch-aware review mode to pi-diff-review by introducing a new base branch scope that compares merge-base(baseRef, HEAD) to HEAD.

The main goal is to make the tool useful not just for working tree review, but also for reviewing the actual delta a branch introduces relative to a chosen base branch.

What's included

• new base-branch review scope
• persisted base-ref config via /diff-review-set-base
• one-off base-ref override via /diff-review <base-ref>
• special default base-ref setting that resolves to the repository's default remote branch
• config inspection via /diff-review-config
• UI updates to:
  • show the active resolved base ref in the window chrome
  • default to the branch review scope
  • default to hidden unchanged regions
  • default to wrap off

Notes on base-ref resolution

When the saved base ref is default, the extension attempts to resolve the repository's default remote branch by:

1. preferring the current branch's upstream remote when available
2. then trying origin
3. then falling back across the remaining remotes

If it still can't resolve a default branch, it asks the user to set one explicitly.

Why this seems useful

Right now the package is great for reviewing:

• worktree changes against HEAD
• the last commit
• all files

But a lot of the time the thing I actually want to review is:

• "what does this branch change relative to main/staging/default branch?"

This adds that path without changing the overall Glimpse + Monaco review workflow.

Implementation details

Files changed:

• src/config.ts — persisted config loading/saving
• src/git.ts — base-ref resolution + merge-base diff/file loading
• src/index.ts — commands, effective config wiring, message rendering
• src/prompt.ts — base-ref-aware prompt text
• src/types.ts — base-branch scope/types
• web/app.js — scope selection/defaults/UI state
• web/index.html — base ref badge + new scope button
• README.md — command/config documentation

Open questions

A few defaults are intentionally opinionated, but easy to change if you'd prefer a more conservative rollout:

• default initial scope is base-branch
• unchanged regions start hidden
• wrap starts off

Happy to split those defaults out or gate them differently if the underlying branch-aware scope/config pieces seem worthwhile but the UX defaults feel too aggressive.

Testing

• npm run check
• local runtime verification against a real repo using the new default base-ref resolution path
• manual verification in pi with the extension loaded via pi -e ...