Add MiniMax image generation extension

[octo-patch]

Reason: MiniMax prompt-driven image generation was not available through the built-in extension runtime.

Changes

• Add regional MiniMax image generation endpoints and an image model registry.
• Persist the MiniMax API key and selected regional origin for server-side extension actions.
• Parse URL and base64 image responses and save generated artifacts in the active workspace.
• Register status and image generation actions with focused coverage.

Checks

• pnpm --filter ipollowork-server exec bun test src/extensions/minimax-image-generation.test.ts src/extensions-connect-gating.test.ts
• pnpm --filter @ipollowork/app exec bun test tests/minimax-provider.test.ts
• pnpm --filter ipollowork-server typecheck
• pnpm --filter @ipollowork/app typecheck
• pnpm --filter ipollowork-server build
• pnpm --filter @ipollowork/app build
• git diff --cached --check
• node .codex/skills/ipollowork-maintainable-code/scripts/audit-changes.mjs (passed with no errors; the adapter-size warning reflects one cohesive runtime owner, and the duplicate-export warnings reference the new file itself)