docs: create assets folders only when needed

[maehwasoo]

What

• Update vault docs so assets/ subfolders are created only when local assets exist.
• Clarify TL;DR/checklist wording for assets placement and creation timing.

Why

• Avoid cluttering the vault with empty assets/ folders (empty directories are not tracked by git).
• Keep guidance aligned with actual usage: create assets/ when you add the first asset.

How

• Update docs/standards/vault/vault-structure.md folder creation rule to be conditional.
• Update docs/standards/vault/assistant-workflow.md TL;DR + checklist to reflect on-demand creation.
• Validation: pnpm check (format:check, lint, typecheck, typecheck:repo, tests)