feat: user-level Cursor plugin install via authenticated download

[rhuanbarreto]

Summary

Replaces the manual Team Marketplace URL flow for Cursor with an automated install that downloads the Cursor plugin tarball from /api/cursor and extracts it into ~/.cursor/plugins/local/archgate/.

This gives Cursor users the full Archgate developer agent, skills (reviewer, lessons-learned, adr-author, cli-reference, onboard), and governance rules — without requiring a Teams/Enterprise plan for Team Marketplaces.

Before

• archgate init --editor cursor was a no-op for plugin delivery — only printed a Team Marketplace URL
• archgate plugin install --editor cursor just printed the URL for manual setup
• Users needed a Cursor Teams/Enterprise plan to use the Team Private Marketplace, or had to rely on the VSIX embedding

After

• archgate init --editor cursor --install-plugin auto-downloads and installs the plugin to ~/.cursor/plugins/local/archgate/
• archgate plugin install --editor cursor directly installs via authenticated download
• Falls back to the Team Marketplace URL if the download fails
• Mirrors the existing opencode install pattern (authenticated tarball → user-scope directory)

Changes

src/helpers/paths.ts

• Add cursorPluginsLocalDir() resolving ~/.cursor/plugins/local/

src/helpers/plugin-install.ts

• Add installCursorPlugin(token) — downloads /api/cursor tarball and extracts to ~/.cursor/plugins/local/archgate/

src/helpers/cursor-settings.ts

• Return user-scope ~/.cursor/plugins/local/ path instead of project-level .cursor/

src/helpers/init-project.ts

• configureEditorSettings cursor case returns user-scope path
• tryInstallPlugin cursor case auto-installs with fallback to marketplace URL

src/commands/plugin/install.ts

• Cursor case calls installCursorPlugin() directly (previously just printed URL)
• Updated printManualInstructions with retry hints + marketplace URL fallback

src/commands/init.ts

• EDITOR_DIRS.cursor changed from "(VSIX)" to "(user-scope)"
• printManualInstructions for cursor updated for new install flow

.cursor/rules/archgate-governance.mdc

• Fixed stale MCP tool references → current CLI commands

Tests

• Updated cursor-settings, init-project, plugin-install, and plugin/install command tests
• Added installCursorPlugin unit tests (download, extract, auth failure, HTTP error)
• Added cursor auto-install + fallback test in init-project

Companion PR

• archgate/plugins — adds /api/cursor endpoint and tarball generation