feat(orch): throttle worker rust builds per target

[nathanwhit]

Summary

Adds a target-local Cargo build throttle for coding workers so multiple workers on the same machine can continue reasoning/editing in parallel without all running expensive Rust builds at once.

• injects a per-target cargo shim for worker-like sessions in both headless and tmux provider modes
• throttles build-like Cargo commands with -max-rust-builds while leaving managers and non-build Cargo commands alone
• adds fail-open behavior: stale slot cleanup, heartbeat refresh, bounded wait via -build-lease-timeout, and unthrottled fallback instead of wedging the fleet
• exposes -cargo-build-jobs, -build-lease-timeout, and -build-lease-stale knobs and documents them

Validation

• go test ./...