feat(editor): support relative paths for git-shared MCP configs (#85)

Workspace-scoped MCP configs (.vscode/mcp.json, opencode.json) are
committed to git, so the absolute path baked into args broke whenever
users switched machines or projects.

- Add PathMode enum (Absolute / ProjectRelative / VSCodeWorkspaceFolder)
  and thread it through GenerateMcpConfigJson, GenerateMcpConfigToml,
  and GenerateOpenCodeConfigJson via a defaulted parameter
- AddToGitHubCopilotConfig now emits ${workspaceFolder}/Library/... so
  VS Code expands the path at launch
- AddToOpenCodeConfig now emits a plain project-relative path
- Add "Use relative path" toggle in the Server Window for the manual
  copy-paste snippet
- Update README with the new examples and a note about re-running
  Configure after UPM package updates (the @<hash> cache segment
  changes)

Per-user configs (Cursor, Windsurf, Claude Desktop, Claude Code,
Codex CLI, Antigravity) keep absolute paths — they live in ~/ and
aren't shared via git.