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/smart-model-selector.ts

@@ -14,6 +14,10 @@ export type ModelAssignment = {
     reviewer: string;
     tester: string;
   };
+  architect: {
+    opus: string;
+    sonnet: string;
+  };
 };
 
 /**
@@ -41,6 +45,7 @@ export async function assignModels(
     return {
       dev: { junior: model, medior: model, senior: model },
       qa: { reviewer: model, tester: model },
+      architect: { opus: model, sonnet: model },
     };
   }
 
@@ -67,6 +72,8 @@ export function formatAssignment(assignment: ModelAssignment): string {
     `| DEV  | junior   | ${assignment.dev.junior.padEnd(24)} |`,
     `| QA   | reviewer | ${assignment.qa.reviewer.padEnd(24)} |`,
     `| QA   | tester   | ${assignment.qa.tester.padEnd(24)} |`,
+    `| ARCH | opus     | ${assignment.architect.opus.padEnd(24)} |`,
+    `| ARCH | sonnet   | ${assignment.architect.sonnet.padEnd(24)} |`,
   ];
   return lines.join("\n");
 }