Modifiable worktree prefix.

[sbansal1999]

We should add a functionality on the app level allowing the user to enter a worktree prefix instead of having it always as t3code.

[lalvarezt]

we might as well tweak the worktree's actual dirname, currently it's just an id and it's very hard at plain sight to know which one you're supposed to open

[serabi]

➕ - to be able to use T3Code for work I need to be able to name the worktrees.

[danielo515]

Same for branch prefix...

[juliusmarminge]

Duplicate of #272

Same ask — make the hardcoded t3code/ worktree branch prefix configurable. Consolidating into #272 (maintainer-acknowledged, with a PR linked).