Add volunteerreminder plugin to the official catalog

[sjuranek]

What this PR does

Adds VolunteerReminder as a remote-source plugin in the official Grok marketplace catalog.

• Plugin name: volunteerreminder
• Type: remote source
• Source URL + pinned SHA: https://github.com/talltimbersgroup/VolunteerReminder.git @ 9e2be257803f48e051db7af526223c6eddb0ee6c
• Homepage: https://volunteerreminder.com/docs/agents

Pin 9e2be257803f48e051db7af526223c6eddb0ee6c is the squash-merge of VolunteerReminder PR 111 onto master (adds repo-root .mcp.json so xAI's generate-plugin-index.py indexes the hosted MCP). Previous pin was 40e7c1d75dea3d8d58ca9b3abc2fd7f2805ea4f1.

Ownership

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

Checklist

• Added/updated exactly one entry in .grok-plugin/marketplace.json (valid JSON, kebab-case name).
• Remote source pins a full 40-char lowercase commit sha, and that commit is public + reachable.
• Regenerated .grok-plugin/plugin-index.json (python3 scripts/generate-plugin-index.py).
• python3 scripts/validate-catalog.py passes locally.
• python3 scripts/generate-plugin-index.py --check passes locally.
• homepage + clear description set.
• License is stated. (Upstream has no root LICENSE file at this pin; happy to add one and bump the SHA.)

Security

• No curl | bash, remote-code download/exec, or postinstall RCE.
• No reading/exfiltration of secrets, tokens, .env, or env vars.
• Hooks and MCP scope are least-privilege. (Remote HTTP MCP only; no stdio/shell MCP, no hooks.)
• Network endpoints this plugin calls (and why):
  • https://mcp.volunteerreminder.com/mcp — hosted Streamable HTTP MCP (roster import / preview in test mode)
  • https://volunteerreminder.com — product app, docs, and import REST API used by that MCP
• Credentials/permissions it requires (and why):
  • User OAuth Allow, or a user-minted API key. This plugin does not ship secrets or headers.

Notes for reviewers

• Official org source: talltimbersgroup/VolunteerReminder.
• Keywords/domains are brand-scoped (volunteerreminder, volunteer reminder; volunteerreminder.com, mcp.volunteerreminder.com). Generic terms were omitted.
• Category omitted: none of the existing catalog categories (development, deployment, monitoring, database, observability) is a close fit, so we did not invent a new one.
• MCP: This pin includes repo-root .mcp.json (mcpServers.volunteerreminder.url = https://mcp.volunteerreminder.com/mcp, no headers/secrets). Regenerated plugin-index.json lists the volunteerreminder HTTP MCP server plus the existing volunteer-schedule-import skill.
• Import is test-mode / preview-first. Nothing texts a real volunteer until a human turns live sends on.