Prepare 0.8.0 watcher reliability release

[pasunboneleve]

Summary

• fix the watch debounce flake by keeping buffered filesystem events out of a cancelable tokio::select! branch
• add narrower watch target handling, including explicit content/-style literal directory targets and a native/poll watcher backend choice
• prepare the 0.8.0 release with updated docs, changelog, development guidance, and test-safety helpers

Testing

• cargo fmt
• cargo test
• cargo clippy --all-targets --all-features -- -D warnings
• ./scripts/ci-smoke.sh
• roborev review HEAD --wait