feat: retain local delivery timing samples

[lipton-agent-hermes]

Summary

• retain exact local-only task and model-call timing samples in the existing private metrics store
• expose local count/p50/p90 summaries
• prune timing samples under the existing 30-day retention policy
• index percentile reads by metric and duration

Privacy and safety

• no prompts, responses, source code, raw IDs, tool arguments/results, or credentials are stored
• shared/exported metric payloads are unchanged
• timing-store errors are caught so task/model completion remains fail-open
• this does not enable telemetry or restart any gateway

Proof

• uv run --extra dev pytest -q tests/hermes_cli/test_relay_shared_metrics.py tests/hermes_cli/test_relay_shared_metrics_runtime.py
• 153 passed
• git diff --check
• fresh privacy/correctness review; retention/index finding reconciled with regression coverage