fix(relay): exempt ephemeral events from Messages quota; add limit_type observability

[wpfleger96]

Problem

Relay WS admission bills every EVENT kind against the per-pubkey durable-message quota (), regardless of whether the event is persisted. With buzz-acp publishing up to 90 observer frames/min + 20 typing indicators/min/channel + 1 presence/min, agents consumed ~111 of their 120/min Messages budget on pure telemetry — leaving only 9 msg/min for real messages and causing repeated 40s quota stalls.

Additionally, agents silently inherited the human WS burst budget with no dedicated config field, and three tier-config fields (agent_elevated_messages_per_min, agent_platform_messages_per_min, agent_standard_api_calls_per_min) were defined, env-loadable, and enforced nowhere.

Changes

Core fix — crates/buzz-relay/src/connection.rs

Ephemeral events (kinds 20000–29999) now skip LimitType::Messages in WS admission. Uses the existing is_ephemeral() range predicate from buzz-core — the same one buzz-db uses to refuse persistence, so admission and storage agree by construction. Ephemeral events still count against WsEvents (per-second burst protection unchanged).

Observability — crates/buzz-relay/src/connection.rs, src/api/bridge.rs

limit_type is now included in:

• WS rejection NOTICE/CLOSED text: rate-limited: quota exceeded (ws_events); retry in 5s
• HTTP 429 response body: rate-limited: quota exceeded (api_calls); retry in 3s
• buzz_admission_rejections_total metric label

The retry in {N}s phrase is preserved intact — ACP and CLI both parse it.

Agent WS budget — crates/buzz-auth/src/rate_limit.rs, crates/buzz-relay/src/config.rs

Added agent_ws_events_per_sec to RateLimitConfig with env override BUZZ_RATE_LIMIT_AGENT_WS_EVENTS_PER_SEC. Default matches the human default (10/s) so this is behavior-neutral at merge. Tune on builderlab once limit_type instrumentation data establishes the right operating value.

Dead config cleanup — crates/buzz-auth/src/rate_limit.rs, crates/buzz-relay/src/config.rs

Deleted three fields enforced nowhere: agent_elevated_messages_per_min, agent_platform_messages_per_min, agent_standard_api_calls_per_min. Removal is grep-clean — no dangling readers in the owned crates.

Tests

New unit tests cover:

• LimitType::as_str() values are stable (breaking change if they change — they appear in metric labels)
• RateLimitConfig::default() has agent_ws_events_per_sec equal to human default
• send_admission_result: NOTICE text names the limit type and preserves retry in Ns phrase for both Messages and WsEvents; Ok(()) sends nothing; sub-scoped rejection emits CLOSED
• BUZZ_RATE_LIMIT_AGENT_WS_EVENTS_PER_SEC env override works and rejects zero

Full suite: 844 passing / 1 pre-existing failure (mesh_demo — reproduces on origin/main before this branch).

Post-deploy validation

After deploy, buzz_admission_rejections_total{reason="quota",limit_type="messages"} for agent pubkeys should drop to ~0. Any residual >5s retry hint on the WS path indicates an unenumerated durable WS publisher.

[wpfleger96]

🤖 Superseded by #4912, which carries the same ephemeral-exemption + limit_type change as part of the consolidated single-PR approach. Branch kept — the agent_ws_events_per_sec config and dead tier-field removal from this PR aren't in #4912 and will follow up separately after it lands.