Add Cursor IDE plugin support

[Mustaffa-Gamall]

Summary

• Shared plugin directory: Added Cursor IDE support to the existing plugin/ directory using a dual-manifest approach — zero file duplication. Both Claude Code and Cursor discover their own manifest (.claude-plugin/plugin.json vs .cursor-plugin/plugin.json) and ignore the other.
• Cursor-specific additions: Only 2 new files inside plugin/: .cursor-plugin/plugin.json (Cursor manifest with marketplace metadata) and rules/archflow-instructions.mdc (Cursor's always-applied workspace rules format).
• Marketplace support: Added .cursor-plugin/marketplace.json at repo root for Cursor Marketplace submission (parallel to the existing .claude-plugin/marketplace.json).
• README updates: Added Cursor installation docs with marketplace install (pending approval) + manual install fallback. Updated file structure to show dual-manifest approach. Updated requirements and contributing sections.

Changes (4 files)

New files

File	Purpose
.cursor-plugin/marketplace.json	Cursor Marketplace registry at repo root
plugin/.cursor-plugin/plugin.json	Cursor plugin manifest (with keywords, repo, homepage, license for marketplace discovery)
plugin/rules/archflow-instructions.mdc	Cursor's always-applied workspace rules (.mdc format with YAML frontmatter)

Modified files

File	Changes
README.md	Added Cursor badge, Cursor Quick Start (marketplace + manual), updated File Structure with dual manifests, updated Requirements and Contributing sections

Why dual-manifest instead of separate directories?

Both Claude Code and Cursor use the same agents, skills, hooks, and phases. The only differences are:

1. Manifest directory name (.claude-plugin/ vs .cursor-plugin/)
2. Cursor adds rules/archflow-instructions.mdc (Claude Code ignores the rules/ folder)

Each IDE discovers its own manifest and ignores the other. This means the shared plugin/ directory works as-is for both — no files need to be duplicated.

Marketplace submission

After this PR is merged, the repo can be submitted to the Cursor Marketplace at cursor.com/marketplace/publish. The .cursor-plugin/marketplace.json at the repo root follows Cursor's multi-plugin repository format.

Test plan

• Verified plugin/ directory contains both .claude-plugin/plugin.json and .cursor-plugin/plugin.json
• Confirmed rules/archflow-instructions.mdc loads correctly as always-applied rule in Cursor
• Tested plugin locally in Cursor — agents, skills, and commands work as expected
• Verified Claude Code plugin still works (.claude-plugin/ unchanged)
• README renders correctly with new Cursor sections