receipts: multi-issuer chain support (v0.2)

[tomjwxf]

Context

v0.1 assumes a single keypair per receipt chain. Real fleets want chains
signed by multiple attestors (e.g. one agent per signer, one HSM per team).

What's needed

• `Receipt` already carries `pubkey` per-receipt, so the format supports
  multi-issuer. The missing piece is the `verify` pipeline.
• `sb verify` should accept a trust-anchor set (`--trust-anchors` flag
  pointing at a JSON file of public keys + role labels).
• Chain rules: every receipt must be signed by an attestor in the anchor
  set; `prev_hash` must chain regardless of issuer change.

Acceptance criteria

• `--trust-anchors` flag
• `verify_chain` handles mixed-issuer chains
• Anchor-set schema documented
• Tests covering: single-issuer, multi-issuer, chain with untrusted
  issuer, chain with revoked issuer

Release target

v0.2.