feat: link agreement events to their on-chain transactions

[MoonBoi9001]

TL;DR

Add on-chain transaction hash fields to IndexingAgreement and IndexingFeeCollection so consumers can audit every agreement state change back to its transaction without grepping indexer-agent logs.

Motivation

The subgraph today captures the terms of each agreement and the timing of every state change, but doesn't store the transaction hash for any of those changes. The mapping has event.transaction.hash in hand on every handler — it just isn't being written to the entity.

This gap was caught during DIPs end-to-end testing on local-network. Recovering the accept transaction hash for a single agreement required SSH-ing into three indexer-agent containers and grepping their JSON logs for the proposalId field. Auditable on-chain links are the whole point of indexing this surface; having them as entity fields on day one closes a clear gap and avoids the same recovery work in every future audit, dashboard, or incident.

Summary

• IndexingAgreement.acceptedAtTx: hash of the accept transaction (typically a multicall).
• IndexingAgreement.canceledAtTx: hash of the cancel transaction (zero sentinel if not canceled).
• IndexingFeeCollection.transactionHash: hash of the collect transaction.
• Defaults use a 32-byte zero sentinel to match the existing canceledBy zero-address pattern.
• Tests extended to assert each new field is captured from the event's transaction hash.

Stacked on #12.