feat(studio): add skill space management and sandbox generation

[evanlowe]

Summary

• add a Skills destination aligned with the existing Studio agent page and support paginated Skill Space management
• add Skill upload, validation, file browsing, deletion, and versioned publishing flows
• add one-to-three independent Dev Sandbox candidates for Skill creation and optimization, with model and style selection
• support Volcengine and BytePlus regions, credentials, default models, and custom model identifiers
• preserve structured upstream errors and automatically repair safe generated Skill format issues before validation

Validation

• pre-commit run
• Python Skill and frontend route tests: 75 passed
• frontend tests: 439 passed
• frontend production build
• real Volcengine Dev Sandbox generation, artifact validation, and automatic frontmatter repair
• Volcengine and BytePlus capability and provider-specific model checks