Add stacktree plugin

[stevysmith]

What this PR does

Adds Stacktree, publishing for agent-generated HTML: one call turns a page into a private, unguessable URL that can be gated, expired, and replaced in place.

• Plugin name: stacktree
• Type: remote source
• Source URL + pinned SHA: https://github.com/stevysmith/stacktree-mcp.git @ 3b56cda36604d75697138bd3221653a28c775103
• Homepage: https://stacktr.ee

Ownership

• I own this plugin or have the right to distribute it.
• The source repo is published under our official org (or I have explained why not below).

Sourced from stevysmith/stacktree-mcp, a personal account rather than an org. I am the author of Stacktree and the sole publisher of the stacktree-mcp npm package (npm maintainer stevysmith <stevy@hey.com>), and the same account owns the entry ee.stacktr/publish in the official MCP Server Registry. Happy to move the source under a stacktree org if you would prefer that before merging.

Checklist

• One entry added to .grok-plugin/marketplace.json, valid JSON, name in kebab-case and unique.
• Remote source pins a full 40-char lowercase commit sha; the commit is public and reachable.
• .grok-plugin/plugin-index.json regenerated and committed.
• homepage, clear description, brand-scoped keywords/domains, and a category.
• Licensed: MIT, stated in the source repo.
• Read the security expectations.

Ran locally, all clean:

python3 scripts/generate-plugin-index.py
python3 scripts/validate-catalog.py          # Catalog OK
python3 scripts/generate-plugin-index.py --check  # Plugin index OK

The diff is additive only: +31 lines, no existing entries touched.

Notes

The plugin exposes the hosted MCP server at https://api.stacktr.ee/mcp (streamable HTTP, OAuth 2.1 with dynamic client registration) plus a stacktree-publish skill. A stdio path exists too via npx -y stacktree-mcp.

Already works in the Grok ecosystem today: asked a Grok Bot to "create a standard intro page and publish it to stacktree" and it published unattended to https://stacktr.ee/p/b0mJDVwkpcWQKZkYPNKR2G/ using the anonymous no-account path.