Add ServiceNow Fluent SDK as external plugin

[bryce-godfrey]

Register the servicenow plugin from servicenow/sdk with a pinned commit SHA and regenerate the component index to include the now-sdk skill.

What this PR does

• Plugin name: servicenow
• Type: remote source
• https://github.com/servicenow/sdk 4aadc235ad57a7442e42529869e68ff19c59596c
• Homepage: https://github.com/servicenow/sdk

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):
• Credentials/permissions it requires (and why):

Notes for reviewers

Maintainer and engineer owner for the ServiceNow SDK at: https://github.com/ServiceNow/sdk