feat: Implement Architect role & design_task tool (#189)

Adds the Architect role for design/architecture investigations with
persistent sessions and structured design proposals.

## New Features

- **Architect role** with opus (complex) and sonnet (standard) levels
- **design_task tool** — Creates To Design issues and dispatches architect
- **Workflow states:** To Design → Designing → Planning
- **Completion rules:** architect:done → Planning, architect:blocked → Refining
- **Auto-level selection** based on complexity keywords

## Files Changed (22 files, 546 additions)

### New Files
- lib/tools/design-task.ts — design_task tool implementation
- lib/tools/design-task.test.ts — 16 tests for architect functionality

### Core Changes
- lib/tiers.ts — ARCHITECT_LEVELS, WorkerRole type, models, emoji
- lib/workflow.ts — toDesign/designing states, completion rules
- lib/projects.ts — architect WorkerState on Project type
- lib/dispatch.ts — architect role support in dispatch pipeline
- lib/services/pipeline.ts — architect completion rules
- lib/model-selector.ts — architect level selection heuristic

### Integration
- index.ts — Register design_task tool, architect config schema
- lib/notify.ts — architect role in notifications
- lib/bootstrap-hook.ts — architect session key parsing
- lib/services/tick.ts — architect in queue processing
- lib/services/heartbeat.ts — architect in health checks
- lib/tools/health.ts — architect in health scans
- lib/tools/status.ts — architect in status dashboard
- lib/tools/work-start.ts — architect role option
- lib/tools/work-finish.ts — architect validation
- lib/tools/project-register.ts — architect labels + role scaffolding
- lib/templates.ts — architect instructions + AGENTS.md updates
- lib/setup/workspace.ts — architect role file scaffolding
- lib/setup/smart-model-selector.ts — architect in model assignment
- lib/setup/llm-model-selector.ts — architect in LLM prompt

lib/setup/llm-model-selector.ts

@@ -15,6 +15,10 @@ export type ModelAssignment = {
     reviewer: string;
     tester: string;
   };
+  architect: {
+    opus: string;
+    sonnet: string;
+  };
 };
 
 /**
@@ -34,6 +38,7 @@ export async function selectModelsWithLLM(
     return {
       dev: { junior: model, medior: model, senior: model },
       qa: { reviewer: model, tester: model },
+      architect: { opus: model, sonnet: model },
     };
   }
 
@@ -70,6 +75,10 @@ Return ONLY a JSON object in this exact format (no markdown, no explanation):
   "qa": {
     "reviewer": "provider/model-name",
     "tester": "provider/model-name"
+  },
+  "architect": {
+    "opus": "provider/model-name",
+    "sonnet": "provider/model-name"
   }
 }`;
 
@@ -119,6 +128,14 @@ Return ONLY a JSON object in this exact format (no markdown, no explanation):
     console.log("LLM returned:", JSON.stringify(assignment, null, 2));
 
     // Validate the structure
+    // Backfill architect if LLM didn't return it (graceful upgrade)
+    if (!assignment.architect) {
+      assignment.architect = {
+        opus: assignment.dev?.senior ?? availableModels[0].model,
+        sonnet: assignment.dev?.medior ?? availableModels[0].model,
+      };
+    }
+
     if (
       !assignment.dev?.junior ||
       !assignment.dev?.medior ||