v0.5.1 — instant session start

Session start no longer blocks on repository size.

• The SessionStart hook now schedules the worktree creation and returns immediately with the promised path — 0.09s on a 43 GB repo (was ~24s).
• A detached creator process builds the worktree: git worktree add --no-checkout (instant), immediate state registration (nothing orphaned if a session dies mid-flight), then a background checkout that clears a .zcode-checkout-pending marker the agent can see.
• Failure hardening from the same field report: kills mid-create can no longer leak orphan worktrees or hold the store lock; a pending-<session> marker makes scheduling idempotent across rapid restarts; creator failures are logged to auto-session-errors.log.
• Context injected at session start now tells the agent how to wait for the background checkout before first file access.