feat(plugin): add hidden --skip-manifest-validation option to pack and upload

[nameless-mc]

Why

There are cases where users need to pack or upload a plugin while temporarily bypassing manifest validation (e.g. debugging, working with a known-invalid manifest, or experimenting with manifest schema changes). Today both plugin pack and plugin upload always run the validator and abort on errors, with no escape hatch.

What

Adds a hidden boolean flag --skip-manifest-validation (default false, not shown in --help) to:

• plugin pack — skips manifest.validate(...) in src/plugin/packer/pack.ts
• plugin upload — skips the manifest validation that runs inside ContentsZip.fromBuffer when reading the plugin zip

To support the upload path, PluginZip.manifest() and ContentsZip.fromBuffer() accept a new { skipValidation } option. The default remains false, so existing callers and existing CLI behavior are unchanged.

The option is intentionally undocumented (hidden: true in yargs) because it is meant for diagnostic use, not for general workflows.

How to test

• Default behavior unchanged: cli-kintone plugin pack / plugin upload against an invalid manifest still fails with the same validation errors.
• With the flag: cli-kintone plugin pack --skip-manifest-validation ... produces a zip without running validation, and cli-kintone plugin upload --skip-manifest-validation ... uploads a zip whose manifest would otherwise fail validation.
• pnpm typecheck, pnpm test, pnpm lint all pass.

Checklist

• Read CONTRIBUTING.md
• Updated documentation if it is required. — intentionally not documented (hidden option)
• Added/updated tests if it is required. (or tested manually) — manual; happy to add tests on request
• Passed pnpm lint and pnpm test on the root directory.