feat: DM-First v2 architecture - Tanko live on CT 112

Architecture change: DM-First (v2) replaces @mention-in-stream (v1) as primary
agent interaction channel. See ADR-001-dm-topology and ADR-002-dm-routing.

Key changes:
- New DM-first Zulip adapter (adapter_dm.py) deployed on CT 112
  - Registers empty narrow to receive ALL events
  - Demuxes DMs (type: 'private') vs stream events
  - Replies to DMs back in private thread, stream replies to #agent-hub
  - Uses blocking long-poll (dont_block=False) to eliminate rate limiting
- Rewritten ARCHITECTURE.md, CONTEXT.md, PRD.md for v2 DM-First
- New ADRs: ADR-001 (DM Topology), ADR-002 (DM Routing)
- Archived old ADRs: 001, 002, 005 (topic-topology, mention-routing, mention-detection)
- Updated ADRs: 003 (agent-naming), 004 (per-agent-bots), 006 (all-bots-model)
- Updated PI extension (index.ts) with DM-aware patterns
- Tanko agent confirmed responding in both DM and @all-bots in #agent-hub

docs/adr/ADR-005-mention-detection.archived.md

@@ -0,0 +1,15 @@
+# ADR 5: @mention Detection via Zulip SDK
+
+**Date**: 2026-04-28
+
+### Decision
+Use the Zulip SDK's `mentioned_users` field in message events for @mention detection, with regex fallback for cleaning @mention artifacts from forwarded message text.
+
+### Context
+The Zulip event API provides `mentioned_users` — an array of user IDs that were @mentioned in the message. This is the most reliable detection mechanism (handles all @mention formats) and avoids fragile text parsing. Regex is only needed to strip the `@**Display Name**` markup from the forwarded message body.
+
+### Consequences
+- Zero false positives for @mention detection
+- Handles @all-bots, @everyone, and individual @mentions uniformly
+- Stripped message text is clean for agent consumption
+- SDK handles reconnection and backoff for the event stream