fix: prune pool-maintenance reaps live unborn-branch upstream config

[zheng-weihao]

Problem

orbit_prune_repo_maintenance judged an orphan branch.<name>.* section by ref existence alone (rev-parse refs/heads/<name>). An empty repo's orbit add bootstraps an orphan worktree whose scoped branch is unborn — registered in the worktree list, upstream config wired, but no ref — so any untargeted orbit prune's closing sweep reaped the live worktree's push routing. The first git push after authoring the first commit then fails 128 (has no upstream branch), breaking scoped mode's wired-up-front promise. Recovery paths are all poor: git's own hint (git push -u origin ws/<ws>/main) publishes the internal prefixed branch name, and the correct repair (hand-editing branch.*) is one the skill's safety rules forbid.

Repro: orbit clone <empty-repo> → orbit add → bare orbit prune → commit → git push → 128.

Fix

Collect branches checked out in worktrees from the same worktree list --porcelain pass (unborn branches are listed there, ref or no ref) and skip their config sections — mirroring orbit_prune_raw_residue's checked-out guard. Two refinements:

• The pool's own checkout does not protect. For an empty repo, porcelain lists the pool checkout under an unborn main; its clone-written branch.<default>.* is bookkeeping residue, not push routing. Exempting the pool (via porcelain worktree→branch line grouping) keeps the previously-correct reap behavior unchanged.
• pwd -P normalization before path comparison. Porcelain prints physical paths (macOS /var → /private/var); a logical $repo never matched the pool's own worktree line, so the pre-existing registration-side pool exemption was latently dead (saved only by the -e fallback). The logical basename is kept for the summary line so a symlinked pool dir stays consistent with its group header.

docs/spec-warnings.md: the orphan-config row now defines the boundary — gone = no ref AND checked out in no non-pool worktree; the pool's own checkout never counts.

Tests

tests/09_prune.bats empty-repo regression pair:

1. bare prune after orbit add keeps branch.ws/dev/main.* while still reaping the pool's branch.main.*
2. done → prune closes the lifecycle: D1 removes the worktree, the guard self-limits, both sections reaped

Full suite: 366/366 passing.