Add typefully plugin

[rajatkapoor]

Official Typefully plugin, sourced from typefully/typefully-plugin at a pinned commit.

What this PR does

• Plugin name: Typefully
• Type: remote source
• Source URL + pinned SHA (remote), or vendored path (local): https://github.com/typefully/typefully-plugin.git @ c265cc23771ea290c860146913138aad92acf011
• Homepage: https://typefully.com

Ownership

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

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; local plugins include README.md + .grok-plugin/plugin.json.
• License is stated.

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.
• Network endpoints this plugin calls (and why): https://mcp.typefully.com/mcp — the hosted Typefully MCP server, the only endpoint the client connects to. The server itself calls Typefully's first-party API and OAuth authorization server on the user's behalf. No other endpoints; both are declared in the plugin README.
• Credentials/permissions it requires (and why): none stored in the plugin. Users sign in via OAuth 2.0. Access follows the user's own Typefully workspace permissions. The plugin ships no hooks, no scripts, and executes no local code — it is an MCP server configuration only.