fix(installer): write Claude project-local MCP config to .mcp.json (#207) (#209)

Project-local installs wrote the MCP server to ./.claude.json, which Claude Code never reads — project-scoped servers must live in .mcp.json. The codegraph tools silently never loaded until users renamed the file by hand. Local installs now write ./.mcp.json and migrate any stale ./.claude.json entry on install and uninstall (siblings preserved). Global installs (~/.claude.json, user scope) were already correct.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>