Adds an opt-in `crewai.integrations.signet` module that produces
Ed25519-signed Signet receipts for every governed action emitted on
the CrewAI event bus: structured tool calls, MCP tool executions,
and A2A delegations.
- Lazy-imports `signet-auth` with a clear ImportError if the
`crewai[signet]` extra is not installed.
- Pairs started/completed events via `started_event_id` so each
receipt covers both the input and the output of a single action.
- Adds configuration surfaces (audit log, policy attestation,
per-surface toggles) via `SignetConfig`.
- Ships tests covering all three event surfaces with a mocked
SigningAgent.
Closes#5568
Co-Authored-By: João <joao@crewai.com>
