fix: batch GitHub PR-state lookups during pruning

[codeforester]

Summary

• batch all branch PR-state checks into one paginated REST lookup
• share the result across local branch, remote branch, and worktree pruning
• retain fail-closed behavior when GitHub lookup fails
• document the lookup and add regression coverage

Root cause

The prune paths performed one GitHub PR query per branch. That was slow and could exhaust the GitHub GraphQL quota before all branches were classified, leaving stale branch entries behind.

Safety

Branches remain retained when the lookup is unavailable or the PR state is unknown. Closed-but-unmerged branches are deleted only with the explicit --closed-unmerged opt-in; open PR branches remain protected.

Validation

• bats cli/bash/commands/basectl/tests/gh-branch-worktree.bats — 37 passing
• bats cli/bash/commands/basectl/tests/gh.bats — 60 passing
• bash -n cli/bash/commands/basectl/subcommands/gh_branch_worktree.sh
• shellcheck -e SC2016 cli/bash/commands/basectl/subcommands/gh_branch_worktree.sh
• git diff --check

Fixes #1951