source-control: worktree-create.sh bare-repo root detection + git ref-syntax validation (PR #898 follow-ups)

[kyle-sexton]

This was generated by AI during triage (#657 sweep housekeeping, T8h batch conversion).

Two valid P2 edge cases on plugins/source-control/scripts/worktree-create.sh, deferred as follow-ups from the 2026-07-22 PR #898 verifier-resolver lane review threads (codex P2, dispositioned + resolved under delegation — deferred as follow-ups, change-freeze on the branch at the time, no fix-repush that cycle). Folded from a #657 line (checked off there, this issue is the batch).

Site 1 — bare-repo roots (in-checkout guard)

A worktree_root under a bare clone (e.g. /repos/other.git/worktrees) makes git rev-parse --show-toplevel fail (no working tree), so the guard falls through and git worktree add creates a checkout inside the bare repo's git dir — violating the "root outside every repository" contract.

Fix direction: also detect a bare-repository ancestor (e.g. probe git rev-parse --is-bare-repository / --git-dir on the nearest existing ancestor) before creating.

Site 2 — git ref-syntax validation (name check)

Names like feat/foo..bar, feat/., foo.lock pass the char-class regex + 64-char limit but git rejects them as refs, so the helper reaches git worktree add and returns environment exit 4 despite documenting an up-front exit 2 for invalid names.

Fix direction: run git check-ref-format --branch after the schema check so the correction flow receives the intended usage error (exit 2).

Acceptance: a bare-repo-rooted worktree_root is rejected before git worktree add runs; an invalid ref-syntax name returns exit 2 (not exit 4). Both sites are mechanical validation-guard fixes; no open design fork.

Work-class: C3 (bug-fix-shaped) — attended triage 2026-07-23, operator-ratified. 🤖

[kyle-sexton]

This was generated by AI on operator direction.

Superseded — this item was already ratified and should not have been queued.

The earlier first-drain ratification request on this issue was raised in error. The admission gate dispatches a C3 bug-fix-shaped item when it carries either its own recorded ratification or a completed first drain — and this issue already carried recorded ratification in its body from attended triage on 2026-07-23. The drain cycle applied the blanket first-drain rule without first reading bodies for an existing ratification, so it queued an item that was already admitted.

Restored to the autonomous frontier: agent-ready re-applied, needs-human removed. No human action is required, and this issue no longer appears in the attended queue's ratify view.

[kyle-sexton]