Add kettle (Developer Productivity)

[ChiHanLu]

Adds kettle under Developer Productivity.

Renamed from claude-sounds mid-review — the name was already taken in the official community directory. Same one-line PR, updated.

The use case: long Claude Code runs mean staring at a terminal. kettle lets you walk away. The part nothing else does: when the turn dies on an API rate limit or overload, a silent terminal looks identical to a working one — I checked the hooks.json of all eleven sound/notification plugins in the community directory and none of them cover StopFailure.

• 11 hook events, each independently switchable: Stop, Notification:permission_prompt, Notification:idle_prompt, SubagentStop, StopFailure, PostToolUseFailure, PermissionDenied, PreCompact, SessionStart, SessionEnd, TaskCompleted
• macOS / Windows / Linux, zero dependencies — afplay / PowerShell MediaPlayer / paplay→ffplay→mpv→aplay
• Portable sound aliases so one config works on any machine; your own files go in ~/.claude/kettle/sounds/
• Random pools, per-event volume, speech output, quiet hours, macOS focus suppression
• Installs silent — nothing plays until the user runs /kettle on
• Playback is detached, hot path returns in ~30 ms, so blocking hooks like Stop never stall a turn
• MIT · claude plugin validate clean · CI selftests on ubuntu/macos/windows · README in English / 繁體中文 / 简体中文

Against the contributing criteria: real use case, does not duplicate anything in the list, and tested (selftest + 3-platform CI).

One line added to README.md, nothing else touched.