Address review: checkpoints, provenance, dual canonicalization

Revise the AI Agent Action predicate based on detailed review feedback:

- Add checkpoint and chain_break as action.type values with sub-schemas,
  addressing the tail truncation detection gap
- Add parties array with witness/asserter roles for field provenance
- Split canonicalization: signing tuple-array (M/L-tagged) for chain
  integrity, RFC 8785 JCS for content digests (float-safe for MCP payloads)
- Document genesis convention (previousHash: "genesis") and chain_break
  requirement for crash recovery
- Add I-JSON safe integer bound on all integer fields (RFC 7493)
- Add 128-level depth bound on extensions with counting rule per in-toto#570
- Replace placeholder digests with fully recomputable worked example
  (every digest has a shown preimage, verifiable against conformance vectors)
- Add Security Considerations section
- Move listing to community contributions per ITE-63 process

Author: elang2