[cruzanstx]

Summary

• scan prompt numbers across the primary checkout and every linked git worktree
• persist the highest allocated number in the shared git directory so removed worktrees cannot cause reuse
• serialize automatic and explicit allocations with OS-backed file locking
• preserve existing folder-scoped lookup behavior

Verification

• python3 -m pytest -q — 668 passed, 1 skipped
• focused prompt-manager suite — 11 passed
• real two-process linked-worktree reproduction allocated 001 and 002 without collision
• independent review findings for stale-lock stealing and explicit-number reuse were reproduced and fixed

Fixes #23