Recognize squash-merged branches during prune

[codeforester]

Summary

• Teach branch/worktree prune to treat merged GitHub PR state as a cleanup-safe signal when Git ancestry fails after squash merges.
• Keep Git ancestry as the first/offline path and keep the conservative skip behavior when GitHub cannot confirm a merge.
• Delete GitHub-confirmed squash-merged local branches with force-delete only after the PR merge signal makes them safe.
• Add BATS coverage for squash-merged branch and worktree cleanup.

Validation

• bash -n cli/bash/commands/basectl/subcommands/gh.sh
• bats cli/bash/commands/basectl/tests/gh.bats
• env -u BASE_HOME HOME=/private/tmp/base-review-home BASE_TEST_PYTHON=/Users/rameshhp/.base.d/base/.venv/bin/python bin/base-test
• git diff --check

Closes #399