feat(agent-metrics): add encrypted usage snapshots - #7443

Summary

• define and validate encrypted NIP-AM kind 44200 agent usage snapshots
• publish context and provider-window usage from local and remote agent paths
• archive and query snapshots with exact agent, channel, and thread scope
• show compact usage rings and details on desktop and Flutter mobile
• bound payloads, provider windows, known agents, history pagination, and caches

Changes

• adds a backward-compatible NIP-AM payload contract with per-field timestamps
• validates NIP-01 event identity/signature, exact p/agent tags, owner/agent binding, forbidden h tags, and encrypted payload sizes
• adds buzz metrics publish for cloud or external producers
• reconstructs scoped mobile history across inclusive same-second pages without silently skipping ties
• preserves partial snapshots with deterministic (timestamp, event id) conflict resolution
• isolates trust/history state across identity and relay changes and bounds remembered scopes
• filters desktop archive candidates in SQL with at most 64 agents and eight newest candidates per agent
• uses profile/local agent names with pubkeys only as fallback
• distinguishes unknown, stale, unavailable, and fresh states without drawing a false 100% ring
• shows the highest known utilization across context and arbitrary provider windows

Testing

• cargo fmt --all -- --check
• cargo test -p buzz-core -p buzz-agent -p buzz-acp -p buzz-cli
• cargo clippy -p buzz-core -p buzz-agent -p buzz-acp -p buzz-cli --all-targets -- -D warnings
• cancel/usage race regression repeated 10/10 successfully
• isolated Tauri archive modules compiled and executed against in-memory SQLite
• dart format --output=none --set-exit-if-changed lib test
• flutter analyze
• flutter test — 2,129 passed
• pnpm check
• pnpm typecheck
• pnpm test — 6,463 passed
• focused desktop status tests — 13 passed
• pnpm build
• differential file-size, whitespace, DCO, forbidden-name, and added-line secret scans
• independent security/correctness review; all findings closed and re-reviewed

Screenshots / recording

Not attached from this Linux build host: GTK/WebKit development libraries are unavailable, so the Tauri application cannot be launched here. Desktop rendering states and mobile interactions are covered by automated component/widget tests; visual confirmation can be added from a supported development machine.

Local environment limitation

The complete native Tauri application could not be compiled or launched because glib-2.0, gtk+-3.0, and webkit2gtk-4.1 development packages are unavailable. To validate the changed native archive code, agent_usage.rs and metric_store.rs were compiled in a minimal Rust harness and executed against in-memory SQLite, confirming the 64-agent request limit, eight-candidate fallback bound, newest-first ordering, and SQL-side agent filtering. Repository CI remains responsible for the full native matrix.