Update xmtp agent guide

[humanagent]

What changed? Why?

Updated Chat Agents documentation to use the correct XMTP repository and current SDK. Original docs pointed to deprecated xmtp/bot-starter repo and outdated @xmtp/xmtp-js SDK.

Key Changes:

• Repository: xmtp/bot-starter → ephemeraHQ/xmtp-agent-examples
• SDK: @xmtp/xmtp-js → @xmtp/node-sdk
• Node.js: v16+ → v20+
• Environment variables: PRIVATE_KEY → WALLET_KEY, added ENCRYPTION_KEY
• Fixed all GitHub example links
• Consistent "agent" terminology throughout

[cb-heimdall]

✅ Heimdall Review Status

Requirement	Status	More Info
Reviews	✅ 1/1	Denominator calculation Show calculation 1 if user is bot 0 1 if user is external 0 2 if repo is sensitive 0 From .codeflow.yml 1 Additional review requirements Show calculation Max 0 0 From CODEOWNERS 0 Global minimum 0 Max 1 1 1 if commit is unverified 0 Sum 1

[soheimam]

hey @humanagent ,

Could you please sign the commits , that way we can merge your changes.

You can read more about it here: https://docs.github.com/en/authentication/managing-commit-signature-verification/signing-commits

[humanagent]

@soheimam thks! just added the signatures