Orbit: permission prompt should pre-select the first option and support keyboard navigation (arrows/numbers)

**Reported via beta feedback (v0.5.1, macOS/arm64).**

> "When the allow/deny popup appears, pre-selecting the first option and allowing to navigate with arrow keys or numbers may be useful to keep hands on keyboard."

The permission prompt (`extensions/loom/exec-guard/gate.ts` -- "Allow once" / "Allow for this session" / "Trust this workspace..." / "Deny") currently needs the mouse. The ask:

- **Pre-select the first option** so Enter is immediately meaningful.
- **Arrow-key navigation** between options.
- **Number keys** (1-4) to pick directly.

Worth pairing with the safety question: pre-selecting a default on a security prompt is a deliberate choice -- "Allow once" as the pre-selected default makes the safe path the fast path only if you consider allow-once safe. Might be better to pre-select without making Enter a blind accept, or to make the destructive/permissive option require an explicit move. Flagging so it's decided rather than defaulted into.

This is prompt-fatigue adjacent: related #233 (progressive/one-time grants), #247 (widen read-only safe-list to cut prompts).

_(Beta tester feedback; details anonymized.)_

Sibling: #376 -- the "Trust this workspace" option in this same prompt currently over-promises for weak-tier models. If its label changes, this option list changes with it.

Start in extensions/loom/exec-guard/gate.ts and trace how the permission prompt builds and handles its four options. Resolve the safe default behavior before implementing pre-selection, arrow-key navigation, and number-key shortcuts, accounting for the related Trust this workspace label change in #376. Done means the prompt supports the requested keyboard interactions without making a permissive choice accidental.

Written by the indexing model from the issue text.