Document OpenProse host capability modes

[rawwerks]

Summary

• document host capability modes in the OpenProse VM spec, including the prose write interactive: false shell wrapper case
• add authoring guidance for explicit mode inputs in ### Requires with mode-specific ### Ensures, ### Errors, and ### Invariants
• clarify that VM primitives like ask_user are not ### Tools, and make prose-author's invocation modes explicit

Validation

• git diff --check
• npm --prefix tools/cli test -- tests/prose/prose-author-contract.test.ts tests/prose/command-model.test.ts tests/cli/cli.test.ts tests/skills/open-prose.test.ts
• safe-push origin codex/docs-host-capability-modes reviewer approved