Add vaultless field to order input/output spec

[findolor]

Motivation

Document the new vaultless field for order inputs/outputs in the YAML spec. This field enables direct wallet-based trading in V6
orderbook.

Parent issue: rainlanguage/rain.orderbook#2402

Solution

Added documentation to ob-yaml.md for the vaultless field:

• Input/Output fields section: Documents required (token) and optional (vault-id, vaultless) fields
• Vaultless mode section: Explains behavior:
  • Vaultless inputs: tokens received go directly to owner's wallet
  • Vaultless outputs: tokens given are pulled from owner's wallet (requires approval)
  • Hybrid orders supported (mix of vaultless and vault-based)
• Validation rules table: Covers error cases for vault-id: 0 and combining vaultless with vault-id
• Examples: Added vaultless-order and hybrid-order examples

Checks

By submitting this for review, I'm confirming I've done the following:

• made this PR as small as possible
• unit-tested any new functionality
• linked any relevant issues or PRs
• included screenshots (if this involves a front-end change)

fixes #43