fix: allow empty string for Expert description field

[FL4TLiN3]

Summary

• Remove .min(1) validation from expertSchema description field to allow empty strings
• Add test case verifying empty string is accepted

Context

The description field in expertSchema had .min(1) validation which rejected empty strings ("") but accepted undefined. This caused issues with UI forms that typically send empty fields as empty strings rather than omitting them.

Before: z.string().min(1).max(1024 * 2).optional()

• ✅ undefined - valid
• ✅ "hello" - valid
• ❌ "" - ZodError

After: z.string().max(1024 * 2).optional()

• ✅ undefined - valid
• ✅ "hello" - valid
• ✅ "" - valid

This aligns with how description fields are handled in skill schemas (McpStdioSkill, McpSseSkill, InteractiveSkill), which use .optional() without .min(1).

Closes #437

Test plan

• pnpm typecheck - passes
• pnpm format-and-lint - passes
• pnpm test - all unit tests pass
• pnpm build - succeeds
• CI passes

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