[Feature Request] Add option to pre-populate editor buffer with previous assistant message as context

**Bug Description**
**Title:** Pre-populate `$EDITOR` buffer with prior assistant message as context when editing

**Body:**

When I hit Ctrl-G to compose/edit a message in `$EDITOR`, the buffer opens empty (or with just my in-progress input). For anything more than a one-liner, I want to reference Claude's last response while I'm writing — quoting specific lines, responding point-by-point, disagreeing with a specific claim, etc.

Right now the workflow is: `/copy` to grab the last response, switch to the editor, paste it as a comment block, then write my reply around it. That friction usually pushes me to just type inline in the TUI and lose the better editing experience.

**Request:** an option to pre-fill the editor buffer with the previous assistant message, commented out (using the comment syntax appropriate to the buffer's filetype, or a configurable prefix like `> `). The commented lines would be stripped on save so they don't get sent as part of my prompt.

**Possible shapes this could take:**
- A setting in `settings.json` (e.g. `"editor.includeLastResponse": true`)
- A separate keybind (e.g. Ctrl-G opens empty, Ctrl-Shift-G opens with context)
- A `PreEditorOpen` hook that lets me control buffer contents myself

Any of these would work — the hook version is the most flexible but the setting is the lowest-friction default.

**Environment Info**
- Platform: linux
- Terminal: kitty
- Version: 2.1.126
- Feedback ID: 3ad66f30-503f-48e4-85d3-e76099ae0f18

**Errors**
```json
[{"error":"Error: Request was aborted.\n at makeRequest (/$bunfs/root/src/entrypoints/cli.js:50:3448)\n at processTicksAndRejections (native:7:39)","timestamp":"2026-07-26T18:00:00.252Z"}]
```

Start by tracing the Ctrl-G flow that opens $EDITOR and review the proposed settings.json location. The issue leaves a setting, keybind, or PreEditorOpen hook undecided, so first clarify the intended interface; done should include the previous assistant message in the buffer, remove the commented context on save, and preserve the user's prompt.

Written by the indexing model from the issue text.