docs: add monitoring & activity audit sections to observability guide
- guide/observability.md: +3 sections (Activity Monitoring, External Tools, Proxying)
- Activity Monitoring: JSONL tool_use audit, jq queries, sensitive pattern detection
- External Tools: ccusage / claude-code-otel / Akto / MLflow / ccboard comparison
- Proxying: NODE_EXTRA_CA_CERTS, ANTHROPIC_API_URL, mitmproxy, Python proxy
- docs: ccboard Activity module implementation plan (Tab 10, Rust models, SQLite cache)
- docs: Mergify cross-system support evaluation
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
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