Add Signet cryptographic signing integration

[willamhou]

Adds Signet to the Integrations section.

Signet signs every CrewAI tool call with Ed25519 and appends to a
hash-chained audit log. Integration is 3 lines:

from signet_auth import SigningAgent
from signet_auth.crewai import install_hooks
agent = SigningAgent.create("my-agent")
install_hooks(agent)

• PyPI: pip install signet-auth
• Apache-2.0 + MIT, fully open source, no commercial services
• 15+ tests for CrewAI integration specifically
• GitHub: https://github.com/Prismer-AI/signet