Support worktree-per-branch workflows

[norswap]

Request: first-class support for repositories where each stack branch is checked out in its own linked worktree (a common setup for multiplexed agent/parallel work).

Today (v0.1.0) this workflow is effectively unsupported:

• Stack-mutating commands (rebase, sync, init adoption) check branches out in their cwd, so any branch held by a linked worktree fails on git's one-branch-one-worktree rule (see gh stack rebase silently succeeds when git rebase --onto fails to start (in branch checked out in another worktree) #35 for a silent-failure aspect of this).
• The local tracking state (.git/gh-stack) is resolved through the per-worktree git dir, so linked worktrees cannot see stacks at all — even gh stack view fails there with current branch "X" is not part of a stack.

What we do today as a workaround, which native support could subsume: wrap gh-stack in a script that temporarily detaches HEAD in every worktree holding a stack branch, runs gh at the main worktree (parking the intended context branch there so stack resolution matches the invoking worktree), and restores everything afterwards.

Possible directions for native support:

• Resolve the state file through the common git dir so all worktrees share it (makes read-only commands work everywhere, cheaply).
• Rebase branches inside the worktree that holds them instead of checking them out (prior art: the --worktrees flag in the boneskull/gh-stack fork), or perform rebases without checkouts and let worktrees fast-forward.

Related bug reports from the same investigation: #471 (recorded-base discarded after parent history edits), #472 (sync exit code on push failure).