`pad mcp install`: add `claude-code` and `codex` targets

[xarmian]

Problem

pad mcp install <client> supports several MCP clients but not the two most common CLI agents — Claude Code and Codex — so users hand-write config for them.

Fix

Add two install targets, mirroring the existing ones:

• claude-code — writes a project .mcp.json entry for the pad MCP server.
• codex — writes the ~/.codex/config.toml MCP server entry.

A docs update on the MCP page is a nice follow-up but not required to close this.

Pointers

• internal/mcp/ (install logic) + cmd/pad/main.go (mcp install).