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/tiers.ts

@@ -7,10 +7,12 @@
 
 export const DEV_LEVELS = ["junior", "medior", "senior"] as const;
 export const QA_LEVELS = ["reviewer", "tester"] as const;
+export const ARCHITECT_LEVELS = ["opus", "sonnet"] as const;
 
 export type DevLevel = (typeof DEV_LEVELS)[number];
 export type QaLevel = (typeof QA_LEVELS)[number];
-export type Level = DevLevel | QaLevel;
+export type ArchitectLevel = (typeof ARCHITECT_LEVELS)[number];
+export type Level = DevLevel | QaLevel | ArchitectLevel;
 
 /** Default models, nested by role. */
 export const DEFAULT_MODELS = {
@@ -23,6 +25,10 @@ export const DEFAULT_MODELS = {
     reviewer: "anthropic/claude-sonnet-4-5",
     tester: "anthropic/claude-haiku-4-5",
   },
+  architect: {
+    opus: "anthropic/claude-opus-4-5",
+    sonnet: "anthropic/claude-sonnet-4-5",
+  },
 };
 
 /** Emoji used in announcements, nested by role. */
@@ -36,6 +42,10 @@ export const LEVEL_EMOJI = {
     reviewer: "🔍",
     tester: "👀",
   },
+  architect: {
+    opus: "🏗️",
+    sonnet: "📐",
+  },
 };
 
 /** Check if a level belongs to the dev role. */
@@ -48,20 +58,29 @@ export function isQaLevel(value: string): value is QaLevel {
   return (QA_LEVELS as readonly string[]).includes(value);
 }
 
+/** Check if a level belongs to the architect role. */
+export function isArchitectLevel(value: string): value is ArchitectLevel {
+  return (ARCHITECT_LEVELS as readonly string[]).includes(value);
+}
+
 /** Determine the role a level belongs to. */
-export function levelRole(level: string): "dev" | "qa" | undefined {
+export function levelRole(level: string): WorkerRole | undefined {
   if (isDevLevel(level)) return "dev";
   if (isQaLevel(level)) return "qa";
+  if (isArchitectLevel(level)) return "architect";
   return undefined;
 }
 
+/** All valid worker roles. */
+export type WorkerRole = "dev" | "qa" | "architect";
+
 /** Get the default model for a role + level. */
-export function defaultModel(role: "dev" | "qa", level: string): string | undefined {
+export function defaultModel(role: WorkerRole, level: string): string | undefined {
   return (DEFAULT_MODELS[role] as Record<string, string>)[level];
 }
 
 /** Get the emoji for a role + level. */
-export function levelEmoji(role: "dev" | "qa", level: string): string | undefined {
+export function levelEmoji(role: WorkerRole, level: string): string | undefined {
   return (LEVEL_EMOJI[role] as Record<string, string>)[level];
 }
 
@@ -74,7 +93,7 @@ export function levelEmoji(role: "dev" | "qa", level: string): string | undefine
  * 3. Passthrough (treat as raw model ID)
  */
 export function resolveModel(
-  role: "dev" | "qa",
+  role: WorkerRole,
   level: string,
   pluginConfig?: Record<string, unknown>,
 ): string {