Worktree hooks name the launch route when quest is missing

When a Claude Code session runs Quest's worktree hooks and quest is not on the PATH Claude Code runs hooks with, the session shows an error saying so and how to fix it, such as launching the agent inside the project's dev shell (nix develop --command claude) or installing quest on that PATH, instead of the shell's bare quest: command not found. A test covers it.

Decided 2026-11-04 (maintainer, /quest-plan).

• The failure happens in the shell, before any Quest code runs, so the fix belongs in the hook entry's command: a guard that checks for quest and otherwise prints the message on stderr and exits 2, which Claude Code shows and treats as blocking. A WorktreeCreate failure then aborts the worktree with that message.
• The entries are a user-level opt-in whose JSON the README's "Claude Code checkouts" section gives and SETUP.md points to, so the guarded entry is the documented one.
• The test runs the documented entry with a PATH that lacks quest and checks the exit status and message, and with quest present checks that the hook runs.