Guard pr merge --delete-branch against worktree conflicts

[tidy-dev]

Closes https://github.com/github/gh-cli-and-desktop/issues/271
Closes #3442

Makes gh pr merge --delete-branch behave safely under git worktrees. Today the local-cleanup half of --delete-branch unconditionally runs git checkout <base-branch> then deletes the branch. In a worktree setup this either fails (fatal: '<base>' is already used by worktree at ...) or silently repurposes the worktree, leaving a confusing partial state.

Changes

Branches the local-cleanup logic on whether the PR head branch is checked out in a linked worktree:

1. cwd IS the linked worktree for the PR head branch - skip local cleanup, print a warning with manual instructions. The merge and remote branch deletion still succeed.
2. PR head branch is in a sibling linked worktree - git worktree remove it, then delete the local branch ref. No git checkout <base> needed.
3. No worktree conflict - existing behavior unchanged.

In all cases gh pr merge never exits non-zero solely because local worktree cleanup could not complete.

Demo

Text transcript of the demo

Three worktrees are set up: main-checkout (main), feature-wt (linked, on wt-demo-scenario1), and sibling-wt (linked, on wt-demo-scenario2).

1. Show starting state
   git worktree list shows all three worktrees.

2. Scenario 1: merge from inside the PR's linked worktree

   cd /tmp/worktree-demo/feature-wt
   git branch --show-current
   wt-demo-scenario1
   
   gh pr merge --squash --delete-branch
   ✓ Squashed and merged pull request tidy-dev/playground#27 (Demo scenario 1)
   ! Branch wt-demo-scenario1 is checked out in the current worktree (/private/tmp/worktree-demo/feature-wt); skipping local delete
     To finish cleanup, first navigate out of the worktree, then run:
     git worktree remove /private/tmp/worktree-demo/feature-wt && git branch -D wt-demo-scenario1
   ✓ Deleted remote branch wt-demo-scenario1
   

3. Scenario 2: merge from main worktree (PR branch lives in sibling)

   cd /tmp/worktree-demo/main-checkout
   gh pr merge wt-demo-scenario2 --squash --delete-branch
   ✓ Squashed and merged pull request tidy-dev/playground#28 (Demo scenario 2)
   ✓ Removed worktree /private/tmp/worktree-demo/sibling-wt
   ✓ Deleted local branch wt-demo-scenario2
   ✓ Deleted remote branch wt-demo-scenario2
   

4. Final state
   git worktree list shows only main-checkout and feature-wt remain - sibling-wt was removed automatically, while feature-wt was preserved with cleanup instructions.

Design decisions

• linkedWorktreeForBranch() skips worktrees[0] - git always lists the main worktree first. By iterating [1:] we only consider linked worktrees, eliminating false positives in normal (non-worktree) repos.
• len(worktrees) > 1 guard - if only the main worktree exists, no linked worktrees are possible so we skip worktree logic entirely.
• No --force on git worktree remove - a dirty worktree is reported as a warning rather than forced, preserving uncommitted work.
• Conventional single-working-directory path unchanged - the git checkout <base> + pull + delete path only runs when there's no worktree involvement.

Test scenarios (4 new + 9 updated)

Scenario	Behavior
cwd is linked worktree on PR branch	Warn + skip local delete
PR branch in sibling linked worktree	Remove worktree + delete branch
Dirty sibling worktree	Warn, skip branch delete
No worktree conflict (normal repo)	Existing behavior unchanged

All 9 existing deleteLocalBranch tests updated with worktree stubs to prevent panics from the new git worktree list call.

[Copilot]

Pull request overview

Updates gh pr merge --delete-branch local cleanup to be safe in repositories using git worktrees, avoiding disruptive git checkout <base> behavior when the PR head branch is checked out in a linked worktree.

Changes:

• Detect whether the PR head branch is checked out in the current linked worktree vs a sibling linked worktree vs no worktree involvement, and adjust local cleanup accordingly.
• Add a git.Client.WorktreeRemove helper and wire it into PR merge local cleanup.
• Expand/update PR merge tests to stub git worktree list / --show-toplevel and add new worktree-specific scenarios.

Show a summary per file

File	Description
pkg/cmd/pr/merge/merge.go	Adds worktree-aware branching to local branch deletion behavior and introduces linkedWorktreeForBranch.
pkg/cmd/pr/merge/merge_test.go	Adds new worktree scenarios and updates existing stubs for new git calls.
git/client.go	Adds a WorktreeRemove helper on the git client.

Review details

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Comments suppressed due to low confidence (2)

pkg/cmd/pr/merge/merge_test.go:2149

• If git worktree remove is invoked with a -- delimiter (to prevent option injection via paths starting with -), this stub needs to include -- so the test continues to match the executed command.

	cs.Register(`git worktree remove /path/to/feature-wt`, 0, "")

pkg/cmd/pr/merge/merge_test.go:2202

• If git worktree remove is invoked with a -- delimiter, this failing stub should also include -- so the test matches the executed command.

	cs.Register(`git worktree remove /path/to/feature-wt`, 128, "fatal: '/path/to/feature-wt' contains modified or untracked files, use --force to delete it")

• Files reviewed: 3/3 changed files
• Comments generated: 4
• Review effort level: Low