Update agent docs to use latest agent-sdk and XMTP patterns

[humanagent]

Following the launch of our new agent-sdk and rethinking the overall developer experience, this PR streamlines the agent documentation:

• Consolidates onboarding - Removes duplicate agent examples across pages for a cleaner learning path
• Updates agent-sdk integration - Uses latest patterns and links to XMTP docs for comprehensive guidance
• Focuses on Base features - Highlights Base-specific Quick Actions while directing to XMTP docs for standard content types
• Improves x402 examples - Updates x402 examples to use newer agent-sdk patterns
• Reduces maintenance - Links to authoritative XMTP docs ensure developers get current information
• Deeplinks: Temporarily removed information about deeplinks in chat as it's not live yet. I have a PR ready to reinstate it once it goes live.

This creates clearer boundaries between Base-specific features and XMTP protocol documentation for a better developer experience.

[cb-heimdall]

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