farm: no per-worktree dependency setup hook (JS/TS node_modules absent in worktrees) #92

Found during the docs-site-mvp /ca:sprint --farm run (2026-06-18).

git worktree add checks out only tracked files, so a JS/TS project's node_modules (gitignored) is absent in each task worktree. Gate commands like npx vitest / npx tsc then fail because the toolchain can't be resolved, and gate commands[0] must stay the narrow test (the mutation guard re-runs it many times), so npm ci cannot live there.

Workaround used: install the toolchain at the project root node_modules (already gitignored). Because worktrees live under .farm/worktrees/<id>/, Node resolves vitest/tsc by walking up to the project-root node_modules. Confirmed working, but it's an undocumented hack.

Perma-fix candidates:

• A one-time-per-worktree setup hook: plan.meta.setup or FARM_SETUP (e.g. npm ci --prefer-offline) run once after worktree add, before gates, and NOT counted in mutation re-runs.
• Or an opt-in "link node_modules from the integration checkout" step for JS/TS plans.
• Document the project-root-node_modules resolution trick in includes/farm.md as the interim pattern.