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-004-per-agent-bots.md

@@ -1,15 +1,12 @@
-# ADR 4: One Zulip Bot Per Agent
+# ADR 4: One Zulip Bot Per Agent (Unchanged)
 
-**Date**: 2026-04-28
+**Date**: 2026-04-28 | **Status**: Still active — DM architecture needs per-agent bots even more
 
 ### Decision
-Each agent has its own dedicated Zulip bot user, co-located on the agent's CT. Each bot runs independently as a platform-native plugin.
+Each agent has its own dedicated Zulip bot user, co-located on the agent's CT.
 
-### Context
-A single bot creates a single point of failure and requires message routing to the correct agent. Per-agent bots provide full isolation — if tanko's bot fails, mumuni's bot continues working. Co-location means zero A2A overhead for the local agent.
-
-### Consequences
-- 6 Zulip bot users to create and manage
+### Consequences (Unchanged)
+- 6 Zulip bot users
 - 6 independent event streams (one per CT)
-- @all-bots requires each bot to know about `All Bots` user
-- Adding a new agent = create Zulip bot + deploy plugin to CT
+- @all-bots still requires each bot to know about `All Bots` user
+- DM isolation: only recipient bot sees the message (improvement over stream-based approach)