chore(agent): Use placeholder values in example config

[nfebe]

Example configuration should ship with inert placeholder values that each operator replaces with their own settings. This updates the remaining concrete values to clearly labelled placeholders.

[sourceant]

Code Review Summary

This PR replaces hardcoded API keys and tokens in the example configuration file with descriptive placeholder values. This is a best practice for open-source repositories to prevent credential leakage and guide users.

🚀 Key Improvements

• Removal of a concrete Google Gemini API key from config.example.yml.
• Replacement of a static internal API token with a prompt for the user to generate a secure string.
• Standardization of placeholder naming across different service sections (PowerDNS, AI, Internal API).

💡 Minor Suggestions

• Adding quotes around placeholder strings in YAML is generally safer to avoid any potential parsing issues with special characters, though not strictly required for these specific strings.

[sourceant]

Review complete. See the overview comment for a summary.

[sourceant]

While replacing the token with a placeholder is correct, for an 'internal_api_token', it is helpful to suggest a specific command or length to ensure users generate a token with sufficient entropy.

Suggested change

	internal_api_token: generate-a-secure-random-string-here
	+ internal_api_token: "<generate-32-char-random-string>"