feat: add type-safe preferences system with AsyncContext

[Gaubee]

Summary

Add a comprehensive type-safe preferences configuration system with AsyncContext integration.

Key Features

1. Preferences Builder API

definePreferences((ctx, p) =>
  p
    .ai((ai) => ai
      .profile("claude-code", (p) => p.useClaudeCodeAgentSdk({
        permissionMode: ctx.env.isDev ? "acceptEdits" : "default",
      }))
      .profile("codex", (p) => p.useCodexAgent({
        modelReasoningEffort: "high",
      }))
    )
    .workflow("git-committer", (w) => w.preferredAgent("codex"))
    .build()
);

2. AsyncContext Integration

• Workflow handlers automatically run within PreferencesContext
• Access preferences anywhere in the call chain without parameter passing
• Uses Node.js AsyncLocalStorage for async context propagation

3. SDK Type Integration

• Direct types from @anthropic-ai/claude-agent-sdk and @openai/codex-sdk
• Type-safe profile configuration

4. Whitebook as Prompt Source

• Read documentation as AI prompts (single source of truth)
• loadWhitebookAsPrompt() utility

Files Changed

File	Description
common/preferences.builder.ts	Fluent builder API
common/preferences.schema.ts	Zod schema definitions
common/async-context.ts	AsyncLocalStorage wrapper
common/whitebook.ts	Whitebook reading utilities
workflows/shared/base-workflow.ts	AsyncContext integration
meta/subflows/preferences.workflow.ts	Renamed from config.workflow.ts
docs/white-book/06-configuration/preferences-api.md	API documentation

Tests

• 89 tests total (all passing)
• Unit tests for builder, schema, async-context, whitebook
• Integration tests for workflow + AsyncContext