feat(acp): finalize DKG memory after agent turns

[branarakic-agent]

Summary

• run a separate, tool-free semantic extraction phase after a successful channel response
• discover the signed Buzz response events and bind them with triggering events as proposal evidence
• sign and submit kind-40009 proposals in the harness rather than asking the model to perform the network mutation
• persist signed proposals before submission and retry the same events every minute after failures or restarts
• keep the user response fail-open and avoid a second chat message

This fixes the observed failure mode where agents answered successfully but never invoked the optional memory CLI. The behavior remains capability-gated and does not change non-DKG relays.

Companion provider idempotency PR: OriginTrail/buzz-dkg-integration (feat/memory-evidence-idempotency).

Verification

• cargo check --locked -p buzz-acp
• cargo test --locked -p buzz-acp --lib (762 passed)
• cargo clippy --locked -p buzz-acp --all-targets -- -D warnings -A clippy::needless_range_loop
• cargo fmt --all -- --check

The single allowed clippy lint is pre-existing on main and unrelated to this change. Signed-off commit included.