refactor: move plugin into plugins/slack/ and add marketplace.json

[WilliamBergamin]

Summary

Restructures the repo so the plugin lives under plugins/slack/ and adds a top-level .claude-plugin/marketplace.json, per the FY27Q2 Technical Specification (Step 5). This enables a Slack-hosted marketplace and a future multi-plugin layout while keeping the plugin's skills, commands, and MCP configuration unchanged.

Layout: the two .claude-plugin/ dirs are deliberate — the root one holds marketplace.json, the plugin's own holds plugin.json.

• Moved (via git mv, history preserved): .claude-plugin/, .cursor-plugin/, .mcp.json, .cursor-mcp.json, skills/, commands/ → plugins/slack/
• Added .claude-plugin/marketplace.json (marketplace slack-skills, plugin sourced at ./plugins/slack)
• Tooling stays at repo root, re-pointed at plugins/slack/ via a PLUGIN_ROOT constant (tests/config.py, scripts/sync_versions.py, scripts/cursor.py) and updated rumdl globs (pyproject.toml). scripts/cursor.py flattens the plugins/slack/ prefix on copy so the local Cursor install matches the root-style layout Cursor expects.
• Docs updated to new paths; fixed the pre-existing stale homepage (slack-mcp-cursor-plugin → slack-mcp-plugin)

Distribution safety

Verified end-to-end with the real Claude Code CLI (v2.1.205): claude plugin validate passes for both the marketplace and the plugin, and /plugin marketplace add ./ + install slack@slack-skills installs cleanly (cache flattened correctly).

The official claude-plugins-official listing is SHA-pinned to the current HEAD, so published installs keep working the moment this merges.

Testing

• make lint — ✅ (rumdl finds all 14 md files at new paths)
• make typecheck — ✅ 16 files, no issues
• make test-unit — ✅ 18/18 (skill discovery resolves new paths)
• python scripts/sync_versions.py — ✅ writes into plugins/slack/ manifests, formatting preserved
• claude plugin validate . and ./plugins/slack — ✅ both pass
• claude plugin marketplace add ./ + install slack@slack-skills — ✅ installs, cache flattened
• make cursor-install — ✅ 16 files land flattened (no plugins/slack/ prefix)

Notes

Follow-ups tracked separately (out of scope here):

• External PR to anthropics/claude-plugins-official: switch the slack entry from a url source to git-subdir with "path": "plugins/slack". Must land before that listing's pinned SHA advances past this restructure, or the official listing breaks.
• Repo rename slack-mcp-plugin → slack-skills (GitHub redirects cover it; never recreate the old name).
• Cursor marketplace listing re-verification for the subdirectory layout.

Requirements

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.
• I've run make test and the tests pass. (make test-unit/lint/typecheck pass; make test-eval requires Gemini keys — runs in CI.)

🤖 Generated with Claude Code