feat!: v3.0 migrate from plugin to MCP server architecture

BREAKING CHANGE: Replace plugin with MCP server

- Replace @opencode-ai/plugin with @modelcontextprotocol/sdk
- New MCP server (src/mcp-server.ts) with STDIO transport
- WebSocket server for Chrome extension communication unchanged
- Screenshot tool now returns base64 by default (optional file save)
- Snapshot tool includes all page links for easier extraction
- No more session lock conflicts between OpenCode instances
- Add AGENTS.md with development guidelines

Migration:
  Remove: "plugin": ["@different-ai/opencode-browser"]
  Add: "mcp": { "browser": { "type": "local", "command": ["bunx", "@different-ai/opencode-browser", "serve"] } }