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-002-dm-routing.md

@@ -0,0 +1,51 @@
+# ADR 2: DM-First Routing
+
+**Date**: 2026-06-21 (supersedes 2026-04-28 @mention-based routing)
+
+### Decision
+
+Agents are triggered primarily by **Zulip Direct Messages (DMs)**. Users send a DM to any agent's bot user (e.g., `@tanko-bot`) to communicate with that agent. Stream-based `@mention` routing is **deprecated** for agent-owner communication but retained for `@all-bots` broadcasts and multi-agent coordination in `#agent-hub`.
+
+### Context
+
+The original design required users to `@mention` an agent in `#agent-hub` stream topics. This proved unreliable because:
+
+1. Zulip 12.0 SDK may not populate `mentioned_users` on stream message events
+2. Every bot subscribed to `#agent-hub` receives every message — wasteful
+3. Topic routing (capturing origin stream + topic for reply) adds parsing complexity
+4. Distinguishing `type: 'stream'` vs `type: 'private'` required extra event filtering
+
+DMs eliminate all of these concerns:
+- Only the recipient bot receives the DM — no broadcast noise
+- No @mention parsing needed — the DM itself is the signal
+- No topic/stream capture needed — reply in the same DM thread
+- `message.type: 'private'` is trivially detectable in the event queue
+
+### DM Event Flow
+
+```
+User sends DM to @tanko-bot
+  → Zulip pushes event to tanko-bot's event queue
+  → Event has: {type: 'message', message: {type: 'private', ...}}
+  → Bot checks: message.type === 'private' (trivial)
+  → Bot reads message.content, processes, responds
+  → Bot sends reply to same DM via /api/v1/messages with
+    {type: 'private', to: [user_id], content: 'response'}
+```
+
+### @all-bots Exception
+
+The stream-based approach is retained **only** for `@all-bots` broadcasts:
+- `@all-bots` in `#agent-hub` → all bots detect the mention and respond
+- This remains necessary for cross-platform broadcasts
+- Detection can use Zulip SDK `mentioned_users` or content-based `@**All Bots**` matching
+
+### Consequences
+
+- **Positive:** Dramatically simpler routing logic — no @mention parsing, no topic/stream tracking
+- **Positive:** Each bot only processes its own DMs — no wasted event processing
+- **Positive:** Zulip's native DM threading handles all reply threading automatically
+- **Positive:** No dependency on `mentioned_users` field behavior in Zulip 12.0
+- **Negative:** Owners must DM their agent directly instead of using stream topics
+- **Negative:** Cross-agent visibility in stream topics is reduced (mitigated by `@all-bots`)
+- **Neutral:** `@all-bots` in `#agent-hub` still uses the original @mention detection pattern