Feat: TUI input changed hook

[brentlopez]

Plugins can now observe TUI input changes in real-time via the new tui.input.changed hook. This enables use cases like intent detection and typing analytics.

• Add tui.input.changed hook type to @opencode-ai/plugin
• Add POST /plugin/input-changed server endpoint
• TUI prompt calls endpoint on input change (fire and forget)
• Add server endpoint test