feat(storage): emit state-change event

[Jagadeeshftw]

Summary

set_agent_allowed and set_agent_blocked write DataKey::AgentAllowed/DataKey::AgentBlocked but were the only per-agent boolean setters that emitted no event — every sibling setter (set_allowlist_enabled, set_require_service_registration, etc.) already publishes a cfg_set event on write.

• set_agent_allowed now emits agt_alw(agent, allowed) after the storage write.
• set_agent_blocked now emits agt_blk(agent, blocked) after the storage write.
• Both topics are 7 characters, well under the symbol_short! 9-char limit, and distinct from every existing topic (no collision).
• No change to fund movement, authorization, or existing return values.

Tests added

• test_set_agent_allowed_emits_agt_alw_event — payload correctness on first call
• test_set_agent_allowed_emits_event_on_every_toggle — a second, independent call emits its own event with the new value
• test_set_agent_blocked_emits_agt_blk_event — payload correctness on first call
• test_set_agent_blocked_emits_event_on_every_toggle — same for the blocklist setter
• test_agt_alw_and_agt_blk_topics_do_not_collide — the two new topics are captured independently and never conflated

Closes #287

Validation

• cargo fmt --all -- --check
• cargo clippy --all-targets -- -D warnings
• cargo test -p escrow — 357 passed (5 new), 1 pre-existing failure unrelated to this change (test_transfer_service_ownership_genuine_transfer_emits_event, fails identically on main)