feat(chat): add multi-agent mode routing scaffold and telemetry.

docs/multi-agent-phase1-runbook.md

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+# Multi-Agent Architecture Phase 1 Runbook
+
+## Scope
+
+This runbook covers mode selection and emergency rollback for:
+
+- `single_agent`
+- `shadow_multi_agent_v1`
+- `multi_agent_v1`
+
+Phase 1 keeps execution behavior on the current single-agent path while mode wiring and telemetry are introduced.
+
+## Resolution Priority
+
+Mode resolution follows this fixed order:
+
+1. Global kill switch (`FORCE_SINGLE_AGENT`)
+2. Request override (`architecture_mode` in chat payload)
+3. System default (`AGENT_ARCHITECTURE_MODE`)
+4. Safe fallback (`single_agent`)
+
+## Configuration
+
+Set environment values in backend runtime:
+
+- `AGENT_ARCHITECTURE_MODE=single_agent` (default)
+- `FORCE_SINGLE_AGENT=FALSE` (default)
+
+Changes require backend restart because config is loaded at process startup.
+
+## Mode Switching
+
+### System default switch
+
+1. Set `AGENT_ARCHITECTURE_MODE` to desired value.
+2. Keep `FORCE_SINGLE_AGENT=FALSE`.
+3. Restart backend.
+4. Verify logs include `[architecture_telemetry]` with expected `architecture_mode`.
+
+### Per-request override
+
+Send optional `architecture_mode` in chat request payload:
+
+- `"single_agent"`
+- `"shadow_multi_agent_v1"`
+- `"multi_agent_v1"`
+
+If `FORCE_SINGLE_AGENT=TRUE`, request override is ignored by design.
+
+## Emergency Rollback
+
+Use the kill switch:
+
+1. Set `FORCE_SINGLE_AGENT=TRUE`.
+2. Restart backend.
+3. Verify new requests log `architecture_mode=single_agent`.
+4. Keep this state until incident is resolved.
+
+## Verification Checklist
+
+- Mode resolves according to the priority order.
+- Kill switch overrides all request/default values.
+- Streaming response schema remains unchanged.
+- Architecture telemetry is emitted with:
+  - `architecture_mode`
+  - `orchestrator_used`
+  - `worker_count`
+  - `retry_count`
+  - `latency_ms`
+  - `token_total`