[SaladDay]

Official API alignment (FS-01/02/03, FS-04/05, FS-12, FS-13/14).

• Skill uploads report field errors: an invalid bundle is invalid_value with param: "files"; a missing files part, default on create and a bad default name their param. default accepts true/false in any case.
• The public upload rejects a name with _ and a description over 1024 characters; installed and plugin Skills are unchanged.
• skills.update body errors name their param; a missing default_version returns 404 with param: "default_version".
• Skill version downloads are application/zip named <name>-<version>.zip.
• Docs (source-files.md + zh) updated.

Checks: go test ./internal/api -run 'Skill|ServeStored|SourceFiles', ./internal/skills/... ./internal/persistence/postgres/skillpg and ./tests/integration -run Skill against a test database, go vet ./..., gofmt.

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