fix(tempo): bind hash credentials to the paying challenge

[raubrey-stripe]

Summary

• Tempo push-flow verification accepted any settled Transfer of the right token, amount, and recipient, so an attacker could present someone else's transaction hash as proof of their own payment (claimOnce only blocked a second use).
• Require a challenge-bound MPP attribution memo (mppx/mpp-rb compatible) unless the merchant set an explicit memo, and parse did:pkh:eip155 credential.source before claiming the hash.
• Ignore local .agents/ skill files.

Test plan

• ./gradlew test
• Confirm a plain Transfer / hash credential is rejected with memo is not bound to this challenge
• Confirm a memo bound to a different challenge or realm is rejected
• Confirm a matching attribution memo verifies, then replay of the same hash still fails
• Confirm a malformed or wrong-chain source is rejected without consuming the replay claim

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