feat(editor): add project-local auto-config for Cursor, Claude Code, Codex CLI (#136)

Adds three new Server Window buttons that write the mcp-unity entry into a
project-local MCP config instead of the user's global config:

- Configure Cursor (Project)      -> <ProjectRoot>/.cursor/mcp.json
- Configure Claude Code (Project) -> <ProjectRoot>/.mcp.json
- Configure Codex CLI (Project)   -> <ProjectRoot>/.codex/config.toml

This prevents MCP connection-failure warnings in non-Unity projects, which
was the original report in #136. All three buttons emit a project-relative
path to Server~/build/index.js so the file is portable across machines when
committed to git (same precedent as GitHub Copilot and OpenCode).

For Codex CLI (Project), the success dialog also reminds the user that
Codex only loads the project config after the project is marked trusted
(first-run prompt).

Existing global buttons (Cursor / Claude Code / Codex CLI) are unchanged.