Add Soon plugin (workforce scheduling, remote MCP)

[OlafJacobson]

What this PR does

Adds the Soon plugin. Soon is a workforce scheduling platform (shifts, rosters, coverage, leave). The plugin bundles a remote MCP server plus three skills, so Grok can read and manage a user's real Soon schedule.

• Plugin name: soon
• Type: remote source
• Source URL + pinned SHA: https://github.com/SoonHQ/soon-claude-plugin.git @ ff4d46be6f70b60a9e2a2c9361e860f88fe68cd5, path: plugins/soon
• Homepage: https://soon.works

Ownership

• I own this plugin or have the right to distribute it.
• The source repo is published under our official org (SoonHQ).

Checklist

• Added exactly one entry in .grok-plugin/marketplace.json (valid JSON, kebab-case name).
• Remote source pins a full 40-char lowercase commit sha; the commit is public and reachable.
• Regenerated .grok-plugin/plugin-index.json.
• python3 scripts/validate-catalog.py passes locally.
• python3 scripts/generate-plugin-index.py --check passes locally.
• homepage + clear description set.
• License is stated — proprietary, in LICENSE at the source repo root. It explicitly permits installing and using the plugin to connect to a Soon account; redistribution and derivative works need written permission.

Security

• No curl | bash, remote-code download/exec, or postinstall RCE. The plugin ships no scripts, hooks, or binaries — it is a .mcp.json, a manifest, a README, and three SKILL.md files.
• No reading/exfiltration of secrets, tokens, .env, or env vars. Nothing reads the filesystem or environment.
• Hooks and MCP scope are least-privilege. No hooks at all. The MCP server is a single remote HTTP endpoint.

Network endpoints this plugin calls (and why): exactly one, https://mcp.soon.works/mcp — Soon's hosted MCP server, which is how the plugin reads and writes the user's schedule. No other host is contacted.

Credentials/permissions it requires (and why): none stored locally. The user authorizes over OAuth at connect time and the server enforces their existing Soon team, role, and board permissions. Tools are scoped (for example schedule:read:all, shifts:write, roles:write) and the connector deliberately exposes no admin, billing, or user-lifecycle tools, so it can plan schedules but cannot reconfigure a workspace. Writes that affect published shifts or other people go through an approval card.

Notes for reviewers

One thing worth explaining up front: the source repo is named soon-claude-plugin because it was first published for the Claude Code ecosystem, and it uses .claude-plugin/plugin.json, which CONTRIBUTING lists as accepted. The plugin body itself is ecosystem-neutral: a remote MCP server plus skills, with nothing Claude-specific in it. Happy to publish under a neutral repo name if you'd prefer that for a Grok listing.

The path: plugins/soon pin is because that repo is itself a small marketplace, so the plugin lives one level down rather than at the root — same shape as the existing browser-use entry.