refactor: Centralize role types and configuration (#190)

Creates a single source of truth for all worker roles via lib/roles/.

## New: lib/roles/

- **registry.ts** — All role definitions (dev, qa, architect) with
  levels, models, emoji, completion results, session key patterns
- **types.ts** — RoleConfig interface
- **selectors.ts** — Query helpers: getRole(), getLevelsForRole(),
  resolveModel(), isValidResult(), roleForLevel(), etc.
- **index.ts** — Barrel exports

## Migrated Files

- **lib/tiers.ts** — Now delegates to registry (backward compat kept)
- **lib/dispatch.ts** — Uses registry for emoji resolution
- **lib/bootstrap-hook.ts** — Uses registry for session key pattern
- **lib/services/tick.ts** — Uses registry for level detection
- **lib/services/heartbeat.ts** — Uses registry for role iteration
- **lib/tools/health.ts** — Uses registry for role iteration
- **lib/tools/work-start.ts** — Uses registry for role enum
- **lib/tools/work-finish.ts** — Uses registry for result validation
- **lib/tools/project-register.ts** — Uses registry for level lists

## Key Benefits

- Adding a new role = add entry to registry.ts (single file)
- No more scattered role unions ("dev" | "qa" | "architect")
- Type-safe role/level/result validation from registry
- Session key pattern auto-generated from registry
- All 64 tests passing (22 new registry tests + 42 existing)

lib/dispatch.ts

@@ -13,7 +13,7 @@ import {
   getSessionForLevel,
   getWorker,
 } from "./projects.js";
-import { resolveModel, levelEmoji } from "./tiers.js";
+import { resolveModel, getEmoji, getFallbackEmoji } from "./roles/index.js";
 import { notify, getNotificationConfig } from "./notify.js";
 
 export type DispatchOpts = {
@@ -302,7 +302,7 @@ function buildAnnouncement(
   level: string, role: string, sessionAction: "spawn" | "send",
   issueId: number, issueTitle: string, issueUrl: string,
 ): string {
-  const emoji = levelEmoji(role as "dev" | "qa" | "architect", level) ?? (role === "qa" ? "🔍" : role === "architect" ? "🏗️" : "🔧");
+  const emoji = getEmoji(role, level) ?? getFallbackEmoji(role);
   const actionVerb = sessionAction === "spawn" ? "Spawning" : "Sending";
   return `${emoji} ${actionVerb} ${role.toUpperCase()} (${level}) for #${issueId}: ${issueTitle}\n🔗 ${issueUrl}`;
 }