docs: add read-aloud plugin - #811

Adds opencode-read-aloud, a TUI plugin that reads OpenCode's latest answer aloud and highlights each word as it is spoken.

• Press ctrl+s to read the answer after the last tool call, and again to pause or resume. Code blocks and tables are skipped.
• The current word and sentence are highlighted in the transcript, which scrolls to keep the paragraph being read on screen.
• Option+click any word, including in older answers, to start reading from there.
• Clickable play/pause, speed and stop controls sit on the prompt row. Speed changes keep the pitch and are remembered.
• On macOS, the AirPods button and media keys pause and resume reading.
• Uses ElevenLabs for the per-word timing, plus ffmpeg. Audio is cached.

npm run validate passes.