Implement per-target MCP server selection/config

[fcimeson]

Goal

Add MCP server selection per install target (project/global) and write the appropriate config.

Notes

• Keep format and behavior TBD until requirements are clarified
• Avoid guessing Claude Code formats; focus on OpenCode-compatible mechanism

Acceptance criteria

• Installer can prompt/select MCP servers per target
• Selected MCP config is written to the correct target
• Behavior is documented