agent: add optional model usage hooks

[longquanzheng]

Summary

• add optional Before/After model hooks with durable Before, Call, After, and Apply Steps
• persist exact per-call ModelUsage while keeping ModelReply unchanged and the legacy no-hooks path intact
• return provider usage separately, add stable CallID retry identity, and make OpenAI response storage explicitly enabled by default
• document the v0.7.0 contract and upgrade path

Verification

• make check-flow-definition
• make test
• make test-race
• DEX_FLOW_SERVICE_ADDRESS=127.0.0.1:8803 make test-dex-integration
• DEX_FLOW_SERVICE_ADDRESS=127.0.0.1:8803 make test-full-stack-e2e
• make check