feat: expand remote workflow telemetry stats

[Sermilion]

Summary

• add proxy-backed remote workflow stats for feature implement and feature verify, including relay capability checks and grouped day/week series
• extend remote implement stats with boundary history metrics and derived usefulness rates
• add feature-verify history relevance/helpfulness telemetry, local stats, and remote relay aggregation
• document the relay contract and cover the new stats surfaces with focused tests

Validation

• .venv/bin/python3 -m unittest tests.test_feature_verify_telemetry tests.test_workflow_stats tests.test_remote_telemetry_stats
• .venv/bin/python3 -m unittest tests.test_mcp_server tests.test_cli tests.test_mcp_stdio
• python3 -m py_compile skill_bill/constants.py skill_bill/db.py skill_bill/feature_verify.py skill_bill/mcp_server.py skill_bill/stats.py skill_bill/sync.py tests/test_feature_verify_telemetry.py tests/test_workflow_stats.py tests/test_remote_telemetry_stats.py
• node --check on docs/cloudflare-telemetry-proxy/worker.js