fix(merge): refuse unresolved conflicts in merge's own name

[max-sixty]

Follow-up to #3579, which added the index gate that keeps conflict markers out of a commit. The gate works on wt merge; it just doesn't refuse in merge's name.

$ git stash pop
CONFLICT (content): Merge conflict in f
$ wt merge --yes
✗ Cannot squash: 1 path with unresolved conflicts
  f
$ wt merge --yes --no-squash
✗ Cannot commit: 1 path with unresolved conflicts
  f

wt merge stages through two different steps, so it borrowed whichever noun it happened to delegate to. Its sibling gate on the same command says Cannot merge, so one command named itself three ways depending on flags.

Threading a caller-supplied noun into handle_squash and CommitOptions would fix the wording, and it's the wrong shape: wt merge already runs ensure_no_operation_in_progress("merge") at its entry, in its own name. Each command gates for itself; the index gate is the one that hadn't been hoisted yet. So it moves up next to its sibling:

repo.ensure_no_operation_in_progress("merge")?;
current_wt.ensure_no_unmerged_paths("merge")?;

The inner guards stay where they are — wt step commit and wt step squash are entry points too, and they keep answering in their own names.

Refusing at the entry also puts it ahead of the approval prompts and the hook plan, rather than partway through the flow.

--no-commit changes which error it hits. It used to reach the dirty-tree check first:

$ wt merge --yes --no-commit
✗ Cannot merge with --no-commit: feature has uncommitted changes
  UU f
↳ Commit or stash changes first

Committing is the one thing that must not happen here, and stashing a conflicted index isn't something git will do either.

Verification

Every entry point, against a real repo stopped on a conflicted git stash pop, with HEAD unmoved in each:

	before	after
wt merge --yes	Cannot squash	Cannot merge
wt merge --yes --no-squash	Cannot commit	Cannot merge
wt merge --yes --no-commit	Cannot merge with --no-commit: … uncommitted changes	Cannot merge
wt step squash --yes	Cannot squash	unchanged
wt step commit --yes	Cannot commit	unchanged

test_merge_refuses_unmerged_paths pins the first row. It's driven from the conflicted-stash-pop helper, which writes no state file, so it can only pass through the index read — and it asserts HEAD is unmoved, not just the message.

Full gate green (4548/4548, lints, fmt, doc sync) on the rebased head, plus cargo clippy --all-targets --features shell-integration-tests.

This was written by Claude Code on behalf of Maximilian