Run heavy commands one at a time in a machine-wide build slot

Several sessions running Rust builds, bun run typecheck and Vitest at once pushed the development Mac’s load average past 100 and starved the local executor. scripts/heavy.sh <command> now runs a heavy command in one lockf-guarded slot shared by every checkout, at nice 10 with CARGO_BUILD_JOBS=6 and VITEST_MAX_WORKERS=4. A waiting command prints who holds the slot.

The first Rust command in a new worktree clones the compiled crates of the most recently built checkout, which turns a 20 to 30 minute cold build into a few minutes. The rule to clean target/debug after every build is replaced by keeping it warm and pruning below 20 GB free.

AGENTS.md lists which commands are heavy, and bun run backend:build builds the backend separately from serving it.