feat: CLI watch mode, batch mode, and proto-ts stub

[kacy]

Summary

• ember-cli watch <key> [interval_ms] polls a key every N milliseconds and prints the value whenever it changes. Exits cleanly on ctrl-c using a pinned signal future so the handler is only registered once
• Batch mode activates automatically when stdin is not a TTY (e.g. ember-cli < commands.txt). Blank lines and # comments are skipped, making it friendly for scripted input
• Added proto-ts to the Makefile with a stub target that prints the TypeScript gRPC codegen command to run once clients/ember-ts/ exists

What was tested

• cargo build -p emberkv-cli passes clean
• cargo clippy -p emberkv-cli -- -D warnings passes with zero warnings
• Watch mode compiles correctly with tokio::select! biased toward ctrl-c shutdown
• Batch mode uses std::io::IsTerminal (stable since Rust 1.70) for TTY detection

Design considerations

• Watch prints only on value change to avoid flooding logs during quiet periods
• The ctrl-c future is pinned outside the loop so it can be polled repeatedly without re-registering the OS signal handler each iteration
• Batch mode treats #-prefixed lines as comments for readability in script files