fix: prune maintenance — the pool HEAD's branch config is not an orphan

[zheng-weihao]

Problem

Pool maintenance's orphan-config reap (orbit prune's closing sweep) judges a branch.<name>.* section purely by ref existence. For an empty repo, the clone-written default-branch section (e.g. branch.master.*) can never have a ref — the branch is unborn by definition — so every bare orbit prune reaps it, even though that section is the pool's live first-push routing (upstream wiring for the first git push), not residue.

The checked-out-branch guard from #33 deliberately left the pool's own checkout unprotected ("stays reapable"). That is fine for genuine residue but misfires on healthy empty repos: the default section is reaped while the repo sits there legitimately unborn.

Change

Exempt the branch named by the pool's HEAD symref (git symbolic-ref --short HEAD) from the orphan-config reap:

• Tracks HEAD, so it cannot leak. The protection self-releases when HEAD moves (pool switches defaults) or when a ref materializes (first push) and the ordinary ref check takes over. The only case kept forever is a HEAD that stays on an unborn branch — which is exactly the live line being protected.
• Invisible in non-empty repos — the ref check already keeps the checked-out default there.
• Name-agnostic — the signal is the HEAD symref, not a hardcoded master/main.
• Documented premise: no orbit command ever detaches the pool's HEAD (sync only fetches; add/switch only touch worktrees), so the symref read always names the default branch; on a detached HEAD the exemption silently no-ops.

Verification

• tests/09_prune.bats 101/101 green:
  • the two empty-repo cases updated to the new criterion (while the worktree lives, both the scoped section and the clone-written default section survive; after the workspace is reclaimed, the scoped section is reaped and the pool default is kept);
  • new case pins the behavior with a non-main default branch (dev).
• Sandbox: two empty remotes (HEAD master / dev) — default sections survive, an injected genuine-orphan section is still reaped, scoped worktree sections untouched.

Docs: docs/spec-warnings.md criterion row and docs/spec-lifecycle.md orphan-config bullet restated to the HEAD-target semantics.