feat(with-watch): use WW_LOG and default diagnostic logs to off

[kdy1]

Summary

• switch with-watch diagnostic tracing configuration from RUST_LOG to WW_LOG
• default with-watch diagnostic logging to with_watch=off while keeping fatal user-facing errors unchanged
• document the new logging contract in the crate README, public docs, and canonical docs

Testing

• pnpm --filter public-docs test
• cargo test -p with-watch
• cargo test