Delete safe GitHub branches during prune

[codeforester]

Summary

• Extend basectl gh branch prune --remote to delete safe GitHub remote branches, not just stale local origin/* tracking refs.
• Keep pruning dry-run by default and require --yes before deleting anything.
• Delete a GitHub branch only when GitHub confirms a merged PR for that exact head branch, including squash merges.
• Keep default/current/worktree-attached branches protected and print separate GitHub branches and Remote tracking refs sections.
• Update GitHub workflow docs and changelog.

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 #410