Optionally confirm merges against the forge to catch squash-merged branches

Problem

Squash and rebase merges rewrite history, so a merged branch is not an ancestor of the default
branch and git branch --merged never sees it. We compensate at effort 3 with
patch_id_match, squash_patch_id_match and merge_adds_nothing (src/branches.rs:192-266).

Those are good heuristics, but they are heuristics. A squash whose commit was amended, reworded
into a different diff, or merged after a conflict resolution produces no matching patch-id, and the
branch stays behind forever. Meanwhile effort 3 is expensive: several extra git invocations per
branch.

The forge knows the answer exactly, and for the squash case it is the only thing that does.

Proposal

Add an opt-in --check-prs (plus sync.checkprs) that, for branches not already proven merged by
the offline strategies, asks the forge whether their pull request was merged, and tags matches as
merged with a distinct reason (BranchReason::PrMerged in src/report.rs).

• Implement via the gh CLI (gh pr list --state merged --head <branch> --json headRefName),
  batched into as few calls as possible rather than one per branch.
• Strictly optional: gh missing, unauthenticated, or offline must produce one warning and fall
  back to the offline strategies — never a failure. This is the same contract as the git >= 2.38
  degradation of merge_adds_nothing.
• Never on by default: it is networked and it leaks branch names to a remote service.
• Classify failures through the existing classify_git_stderr / GitErrorKind machinery so
  network and auth errors are distinguished.

Open question worth settling in this issue: whether to shell out to gh at all, or to keep the
tool forge-agnostic and skip this entirely. A gh dependency is a real cost for a tool that
currently needs nothing but git.

Acceptance

• Squash-merged branch that defeats the patch-id strategies is detected with --check-prs.
• gh absent / logged out / offline → warning, run completes, offline result unchanged.
• Off by default; documented as networked and opt-in.
• pr-merged distinguishable from merged in text and JSON output.

Start in src/branches.rs:192-266 and inspect src/report.rs for existing merge strategies and BranchReason values. Review the classify_git_stderr/GitErrorKind machinery before deciding how optional gh calls should degrade. Done means an opt-in --check-prs and sync.checkprs detect qualifying merged PRs, remain off by default, warn and fall back when gh fails, and expose pr-merged in text and JSON.