docs(adr): RFC — Audit Log & Usage Dashboard

[masami-agent]

Summary

Proposes a pluggable audit event system for OpenAB with configurable sink adapters and an optional dashboard container.

Key Design Decisions

• Pluggable sinks — file (JSONL), S3, OTLP, webhook; users choose where data goes
• Privacy-first — no prompt/response content stored by default; metadata only
• Separate dashboard container — openab-dashboard reads audit data, provides Web UI
• 3-phase implementation — Phase 1 delivers file sink + instrumentation; Phase 2 adds S3/webhook; Phase 3 adds OTLP + dashboard

Open Questions (seeking feedback)

1. Should audit events include thread_id for conversation correlation?
2. Cost attribution: estimate dollar cost per session, or leave to dashboard?
3. Real-time streaming vs batch query for dashboard MVP?
4. Multi-tenant data partitioning strategy?

Related Issues (to be opened after RFC approval)

8 implementation issues drafted — covering trait foundation, file/S3/OTLP/webhook sinks, lifecycle instrumentation, dashboard container, and docs.

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cc @thepagent for review