Local git-branch switch (checkout, block-on-dirty)

[pechhe]

Parent

#38

What to build

Local git-branch switching from the Environment widget's git-branch row. In Local Location, the row lists git branches; selecting one checks it out in the primary checkout. If the working tree is dirty, the switch is blocked with a clear message — no auto-stash. (Worktree threads keep the read-only base/detached display from #40.)

Acceptance criteria

• In Local mode, the git-branch row lists branches and switches (checkout) on select.
• A dirty working tree blocks the switch with a clear, actionable message.
• No auto-stash occurs under any condition.
• Worktree-mode branch row remains read-only (base + detached/real name).
• core-lane test asserts checkout-on-select and the dirty-tree block message.

Blocked by

• Environment widget: read-out + popover #40