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/workspace.ts

@@ -10,6 +10,7 @@ import {
   HEARTBEAT_MD_TEMPLATE,
   DEFAULT_DEV_INSTRUCTIONS,
   DEFAULT_QA_INSTRUCTIONS,
+  DEFAULT_ARCHITECT_INSTRUCTIONS,
 } from "../templates.js";
 
 /**
@@ -49,6 +50,11 @@ export async function scaffoldWorkspace(workspacePath: string): Promise<string[]
     await fs.writeFile(qaRolePath, DEFAULT_QA_INSTRUCTIONS, "utf-8");
     filesWritten.push("projects/roles/default/qa.md");
   }
+  const architectRolePath = path.join(defaultRolesDir, "architect.md");
+  if (!await fileExists(architectRolePath)) {
+    await fs.writeFile(architectRolePath, DEFAULT_ARCHITECT_INSTRUCTIONS, "utf-8");
+    filesWritten.push("projects/roles/default/architect.md");
+  }
 
   // log/ directory (audit.log created on first write)
   const logDir = path.join(workspacePath, "log");