🤖 refactor: unify branch name input with autocomplete

Replace separate 'New branch' / 'Existing branch' toggle with a single
combobox-style input that:

- Shows matching existing branches as user types (autocomplete dropdown)
- Selecting a branch from dropdown → uses existing branch, hides 'from main'
- Typing a non-matching name and pressing Enter → creates new branch
- Auto-generation via magic wand still works when input is empty

Benefits:
- Simpler, more intuitive UX - no mode switching required
- Natural discovery of existing branches while typing
- Consistent behavior whether creating or reusing branches

Technical:
- New BranchNameInput component with Popover-based autocomplete
- CreationControls simplified (removed mode toggle, branch picker integration)
- Trunk branch selector hidden when existing branch is selected

Signed-off-by: Thomas Kosiewski <[email protected]>

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_Generated with `mux` • Model: `anthropic:claude-opus-4-5` • Thinking: `high`_

Change-Id: I1dd55befbd7c2902fc2ae7e095925776e6534504

Author: ThomasK33