Add contextstream plugin

[escott-]

What this PR does

• Plugin name: contextstream
• Type: remote source
• Source URL + pinned SHA: https://github.com/contextstream/grok-plugin.git @ 25e5abade0cb07b0ce6df155396cc3620793645a
• Homepage: https://contextstream.io

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).
• homepage + clear description set; local plugins include README.md + .grok-plugin/plugin.json.
• License is stated (MIT on the plugin repo).

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.contextstream.io/mcp only — official ContextStream hosted MCP for project memory (decisions, lessons, session recall, search).
• Credentials/permissions it requires (and why): sign-in on first connection to the hosted MCP. No API key in the catalog. No hooks.

Notes for reviewers

Official ContextStream org plugin. Thin packaging: .grok-plugin/plugin.json, .mcp.json (type http), one skill, MIT LICENSE. No hooks, no postinstall, no local binary. Category development to match live catalog + other hosted-MCP plugins (Tavily, Firecrawl). Keywords are brand-scoped (contextstream, contextstream memory, contextstream mcp) to avoid generic memory CTA collisions.