Add customizable worktree branch naming

[benmarg]

What Changed

There's now a dropdown in the bottom of the editor that allows you to select between three different Branch Naming modes: Auto, Prefix, and Custom

"Auto Branch Name" is the same as the current behavior
"Prefix" allows you to change the prefix from t3code to whatever you'd like, so for example "feat". T3code will still rename the part after the prefix when you go to create a PR.
"Custom Branch Name" allows you to totally override the branch name and do whatever you'd like with a static branch name

The menu goes away after you create the thread

Side note:

Adding tests bulked up the line count so it's a bit of a bigger PR but please don't ban me 😅, just close the PR if you think it's too big

Why

Using the default t3code prefix isn't always desired. A key workflow this is blocking for me is using the default branch name that Linear provides when creating a worktree, so the branch can automatically be tied to the ticket by Linear.

UI Changes

Previous:

New:

Screen.Recording.2026-03-14.at.11.40.55.PM.mov

Checklist

• This PR is small and focused
• I explained what changed and why
• I included before/after screenshots for any UI changes
• I included a video for animation/interaction changes

Note

Add customizable worktree branch naming with auto, prefix, and full modes

• Introduces a GitWorktreeBranchNaming contract schema (auto/prefix/full modes) in git.ts and shared utilities in shared/src/git.ts for building and detecting temporary branch names.
• Adds a branch naming selector UI to BranchToolbar.tsx so users can choose auto, a custom prefix, or a fully explicit branch name when creating a new worktree.
• Persists the chosen naming config through the full stack: draft store → thread.create command → thread.created event → projection_threads.worktree_branch_naming_json column (added in migration 014).
• Replaces inline branch detection logic in ProviderCommandReactor.ts with the new shared buildFinalWorktreeBranchName and isTemporaryWorktreeBranchName utilities.
• Behavioral Change: existing rows are backfilled to {"mode":"auto"} by the migration; the repository now emits decode errors if the stored JSON is invalid.

^{Macroscope summarized d0e6cec.}

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