feat(code): unified PR creation workflow

[adboio]

Problem

the current method for getting changes shipped takes a lot of clicks, and at least 3 separate modals -- create branch, commit, create PR.

for what i expect to be the more general case ("agent did work, i wanna ship it"), we can simplify this

Changes

introduces a new unified "create PR" workflow that does these steps, as needed:

1. create & checkout new branch
2. commit
3. push
4. create PR

notes:

• if the user is already on a non-default branch, the first step is optional
• if the user has no uncommitted changes, the flow starts on step 3
• if a PR already exists on the current branch and there are new uncommited changes, the flow creates a new branch (stack time baby)
• AI generation is delayed til the user clicks "create" - happens if the commit msg and/or pr details are left empty
• primary CTA swaps to "view PR" when a PR exists, until new changes appear, then it swaps back to "create PR"
• all existing individual actions are still available in the dropdown

other minor fixes:

• increases codemirror's default diff scan limit to avoid wonky diff rendering
• adds dupe checking to auto-suggested branch names
• updates pr title/body generator to pass the whole diff instead of just filenames

How did you test this?

i have not yet tested this in cloud or worktrees

manually tested, i know it's a big PR, so would appreciate others manually testing a bit too 💪

[adboio]

• feat(code): unified PR creation workflow #1377 👈 (View in Graphite)
• main

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