docs(ai-agents): add NGZ AgentKit skill page for onchain habit accountability

[bennytimz]

What changed?

Adds a new AgentKit skills page for No-Going-Zone (NGZ), an onchain habit accountability tracker deployed on Base Sepolia.

New file: docs/ai-agents/skills/agentkit/ngz.mdx
Updated: docs/ai-agents/skills/index.mdx — added NGZ card to the skills overview
Updated: docs/docs.json — added new AgentKit subgroup under the Skills nav with the NGZ page

Why?

The Skills section covers wallets, payments, and trading but has no AgentKit subgroup, even though AgentKit is Coinbase's own agent toolkit with 50+ action providers. The NGZ action provider was just added to coinbase/agentkit in PR #1126 and is a concrete working example of a read/write action provider built on a real deployed Base contract.

Adding an AgentKit subgroup here:

1. Closes a gap in the Skills section — developers using AgentKit have nowhere to go in the AI Agents docs
2. Provides a real working example of an action provider (not a placeholder)
3. Creates a natural home for future AgentKit action provider skill pages

Notes to reviewers

• Follows exact formatting of existing skills pages (frontmatter, Install → coverage table → prompts → reference)
• Card icon uses fire from the Mintlify icon set (matches the habit/streak theme)
• No pages removed, no redirects needed
• NGZ contract is on Base Sepolia testnet — noted in a <Note> callout on the page

How has it been tested?

Pages follow the same MDX structure as existing skills pages. The action provider referenced is live on coinbase/agentkit PR #1126 and all 16 tests pass locally.

[cb-heimdall]

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