feat(worktree): portable create-worktree.sh + Bug 1 base-ref fix (I-phase)

[artyhoo]

What

I-phase for the dual-channel worktree-create capability (verdict combo b+c, R-phase merged PR #297 / patch 2026-05-30-worktree-create-dual-channel.md).

• (b) BUILD scripts/create-worktree.sh — portable, CLI/CI/non-CC-agent-callable worktree setup. <name> [<project-dir>] [<base-ref>]; mirrors the CC hook's semantic check, reads CLI args instead of JSON stdin, no jq dep. Symlinks node_modules per the D2 workspace-optimisation pattern.
• Bug 1 fix in .claude/hooks/worktree-setup.sh — git remote set-head origin --auto before the base-ref loop. A stale local origin/HEAD (frozen at main, pre the 2026-05-22 staging migration) silently based new worktrees on the wrong branch.
• (c) REFERENCE Superpowers using-git-worktrees (SSOT feat(skill-drift): multi-channel drift detection from D-AuditC-5 #65) for the AI-session axis — unchanged; BUILD serves the orthogonal human/CI axis.

Cold-QA findings from the R-phase, addressed

• Finding 1 (configurable trunk): base-ref resolution is explicit arg → $WORKTREE_BASE_REF → refreshed origin/HEAD → fallback chain. No hardcoded origin/staging — refreshing origin/HEAD resolves to whatever the remote's actual default is (staging here, main for a consumer), so it's portable without knowing the name.
• Finding 2 (marker replace, not delete): hook's @cc-only-rationale → @dual-pair: worktree-create-setup. A portable equivalent now exists, so @cc-only-rationale is factually wrong (dual-implementation-discipline.md §6). This is the first dual-channel pair in the repo.

Empirical evidence

Bug 1 reproduced live this session:

$ git symbolic-ref refs/remotes/origin/HEAD          → refs/remotes/origin/main   (stale)
$ git remote set-head origin --auto && git symbolic-ref refs/remotes/origin/HEAD → refs/remotes/origin/staging  (fixed)

Tests (paired-negative discipline, principle 02)

• create-worktree.test.ts — 11 cases: happy path, both symlinks, branch naming, idempotency, single-line stdout, explicit base-ref, $WORKTREE_BASE_REF, default project-dir, + 2 paired-negatives (missing name, non-existent base-ref).
• worktree-setup.test.ts — new BUG-1 regression building a temp repo with a deliberately-stale origin/HEAD; fails on the unfixed hook (worktree based on main), passes on the fix (staging).

Verification

• npx vitest run worktree-setup + create-worktree + check-hook-marker → 29 passed
• All principle meta-tests → 157 passed (also green in pre-push)
• shellcheck both scripts → clean
• Pre-existing baseline tsc error in audit-self/audit-ai-docs.ts:67 (remark types) is unrelated and present on origin/staging.

Scope-out / notes

• scripts/create-worktree.sh is not shipped to consumers via install.sh (repo-internal dev tooling) and is outside packages/, so it's not a capability commit by the pre-push detector — the Prior-art: trailer is added per patch §5 mandate regardless.
• The dual-implementation §5 drift-grep scans agents/ + .claude/skills/ for a @dual-pair counterpart, not scripts/. No CI gate exists for it (promotion deferred per §9), so no mechanical failure; flagging for awareness if that grep is ever promoted.