Edit queued prompts in-place before they execute

### Describe the feature or problem you'd like to solve

Once a prompt is queued it is immutable. If you realise a queued prompt needs a correction — a missing detail, a wrong file path, a scope change — your only option is to remove it and re-type it from scratch. There is no way to open and edit an already-queued entry.

Related: #1683 (reorder), and the companion remove issue (#1781) — this is specifically about **in-place editing** of queued prompts.

### Proposed solution

From the interactive queue manager (or via the same `Ctrl+Q` / `/queue` entry point), add an **edit action** on any queued item:

- **`e` or `F2`** on a selected queue entry opens it for editing — either:
- **Inline**: the prompt text becomes an editable field in the TUI, same as the main input bar, with full cursor navigation
- **External editor**: respects the `$EDITOR` environment variable (same as `Ctrl+G` in the main prompt bar)
- On **Save / Enter**: show a one-step confirmation diff:
```
Update queued prompt?
Before: "Refactor the auth module"
After: "Refactor the auth module and add JSDoc comments"
Confirm? (Y/n)
```
- On **Escape**: discard edits, return to queue list unchanged.

The confirmation diff view is important — it makes it impossible to accidentally overwrite a queued prompt without seeing exactly what changed.

### Example prompts or workflows

1. User queues "Fix the login bug in auth.ts" then realises the file is `auth.service.ts`. Opens queue manager, edits the entry, confirms — no re-typing needed.
2. User queues a vague prompt. While the current agent is running, user refines the queued prompt with more context before it executes.
3. User has `$EDITOR` set to VS Code or Neovim — pressing `e` on a queue entry opens it there, same as `Ctrl+G` works for the live input bar.
4. User accidentally presses `e`, makes unwanted changes, hits Escape — original prompt is fully preserved.

### Additional context

- OS: Windows 11 Pro (Build 26200 / 24H2)
- Shell: PowerShell 7.5.4
- Copilot CLI: 0.0.420 (win32-x64)
- The `$EDITOR` environment variable and `Ctrl+G` shortcut (added in 0.0.419) already support external editing for the live input bar — queue editing should respect the same behaviour for consistency.