Add packSkill() to SDK for building .skill bundles programmatically

[mgoldsborough]

Summary

The CLI has skill pack which validates a skill directory and creates a .skill ZIP bundle with SHA256. This is currently in packages/cli/src/commands/skills/pack.ts. Promoting it to the SDK enables CI/CD pipelines and platform tooling to build skill bundles without the CLI.

Proposed API

interface PackSkillResult {
  success: boolean;
  name?: string;
  version?: string;
  path?: string;
  sha256?: string;
  size?: number;
  error?: string;
}

// On the Mpak facade or as a standalone utility:
async packSkill(skillDir: string, outputPath?: string): Promise<PackSkillResult>

Acceptance Criteria

• Validates skill directory before packing (reuses SDK validation from Add skill cache management to SDK (list, remove, validate installed skills) #75)
• Creates .skill ZIP bundle with correct directory structure
• Computes SHA256 hash
• Returns metadata (name, version, path, sha256, size)
• Unit tests
• CLI skill pack updated to use SDK method

Priority

Lower than #74 and #75 — this is a build-time operation, not runtime. But completes the full skill lifecycle in the SDK.