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/model-selector.ts

@@ -51,7 +51,7 @@ const COMPLEX_KEYWORDS = [
 export function selectLevel(
   issueTitle: string,
   issueDescription: string,
-  role: "dev" | "qa",
+  role: "dev" | "qa" | "architect",
 ): LevelSelection {
   if (role === "qa") {
     return {
@@ -60,6 +60,17 @@ export function selectLevel(
     };
   }
 
+  if (role === "architect") {
+    const text = `${issueTitle} ${issueDescription}`.toLowerCase();
+    const isComplex = COMPLEX_KEYWORDS.some((kw) => text.includes(kw));
+    return {
+      level: isComplex ? "opus" : "sonnet",
+      reason: isComplex
+        ? "Complex design task — using opus for depth"
+        : "Standard design task — using sonnet",
+    };
+  }
+
   const text = `${issueTitle} ${issueDescription}`.toLowerCase();
   const wordCount = text.split(/\s+/).length;