swtich to nearest surviving branch after modify

[skarim]

After gh stack modify applies changes, the user may end up on an
orphaned branch that is no longer part of the stack — for example, if
their checked-out branch was dropped, folded into another branch, or
renamed. Previously, the code blindly restored the original branch
regardless of whether it still existed in the stack.

Add a resolveCheckoutBranch helper that inspects the modify plan and
the post-modify stack to determine the best branch to check out:

1. Still in stack → keep the original branch (no-op)
2. Renamed → check out the new name
3. Folded down → check out the fold target (branch below)
4. Folded up → check out the fold target (branch above)
5. Dropped → check out the nearest surviving neighbor
   (prefer above, fall back to below)
6. Fallback → topmost branch in the stack

Both ApplyPlan and ContinueApply (the --continue path) now use
this helper instead of unconditionally restoring the original branch.
When the resolved branch differs from the original, a message is
printed so the user knows they've been switched.

The resolution uses the pre-modify snapshot (already persisted in the
state file) to determine original adjacency, so it works correctly even
when multiple branches are removed in the same operation.

Includes 12 new tests:

• 9 unit tests for resolveCheckoutBranch covering all action types,
  edge cases (topmost dropped, multiple drops, empty stack), and
  the fallback path
• 3 integration tests verifying ApplyPlan checks out the correct
  branch after drop, fold-down, and rename operations

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