…#34)

* fix: branch dropdown, diff base branch, and direct mode auto-checkout

Three bug fixes for the task creation and diff review workflows:

1. Branch dropdown: merge two racing SolidJS createEffect blocks into one
   effect with loading spinner and retry+toast error handling (DROP-01, DROP-02)

2. Diff base branch: normalize empty-string baseBranch to undefined at the
   IPC boundary (register.ts), task creation (tasks.ts), and legacy state
   load (persistence.ts) so diffs use the correct per-task base branch
   instead of silently falling back to main (DIFF-01, DIFF-02, DIFF-03)

3. Direct mode auto-checkout: replace "Please checkout X first" warning with
   automatic git checkout when creating a Direct mode task on a non-current
   branch (CHECKOUT-01)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* adding checkout branch

---------

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

When detectMergeBase could not compute a merge-base for any ref, it
returned the branch name (e.g. "main"). This caused git diff to compare
against main's current tip rather than the fork point, making every file
changed on main appear in the task's Changed Files panel.

Return headRef instead so the diff is HEAD vs HEAD (empty, safe default)
and skip caching the fallback to allow retrying on the next call.

* task/if-master-branch-has-commits-not-in-the-0c4159:
  fix(git): fall back to HEAD instead of branch name when merge-base fails

…ata loss

When an AI agent checks out a different branch in a worktree, the merge
dialog would silently merge the stale original branch, discarding work.
Now detects mismatches (wrong branch or detached HEAD) and blocks the
merge with an actionable "Use current branch" button to realign the task.

Diff changed files against main's current tip instead of the merge-base,
filtered to only files the feature branch touched. This eliminates
"phantom" files that both branches changed identically and shows only
what would actually change in main when merged.

When main hasn't moved ahead (common case), behavior is unchanged.