feat: add optional MuAPI image provider

[Anil-matcha]

Summary

Adds an opt-in muapi provider to the generate-image command.

• Supports text-to-image and reference-image editing through MuAPI.
• Uploads local references, submits the job, polls the bounded result endpoint, and saves the returned image locally.
• Validates HTTPS media URLs and image signatures before writing output files.
• Keeps the existing default provider and adds the provider documentation to both bundled skill variants and the skeleton.

Verification

• corepack pnpm --filter @cluesmith/codev exec vitest run src/__tests__/generate-image.test.ts — 39 passed
• corepack pnpm --filter @cluesmith/codev build — passed
• Tests use mocked HTTP responses; no live or billable generation request was made.

MuAPI setup: access keys and image API.