Feature request: Add Selection to Conversation for chat responses (like Codex)

### Problem

When a user selects text **inside a Copilot chat response** (or a previous chat message), there is currently no built-in way to add that selected text back into the current chat input as context — unlike OpenAI Codex, which shows a floating **"Add to conversation"** button (and a context-menu item) whenever text inside the conversation is selected.

Currently in VS Code:

- `Add Selection to Chat` (`github.copilot.chat.attachSelection`) only works for **editor text / files**; it does nothing when the selection is inside a chat response.
- Selecting text in a chat response only offers **Copy / Copy as Markdown**; there is no "Add to conversation" affordance (neither floating button nor context-menu item).
- Chat input drag-and-drop only accepts files, folders, images, URLs, symbols, problems, SCM changes, or whole-chat references — **plain text selected from a chat response cannot be dropped in**.
- To quote or reference part of a previous response in the same conversation, users must manually **copy & paste**, which is disruptive.

### Solution

Add a **"Add Selection to Conversation"** affordance for text selected inside chat responses / messages, matching the Codex UX:

1. When the user selects text inside a chat response, show a small **floating action bar** (like Codex) with an action such as **"Add to Conversation"**.
2. Also add a **context-menu item** ("Add Selection to Conversation") when right-clicking selected chat response text.
3. Clicking it should attach the selected text to the **current chat input** as a quoted/context reference (e.g. as a `#`-mention-style context chip or an inline quoted block), so the user can keep typing in the same conversation without copy-paste.

The existing **"Ask Question"** (side chat) behavior on selection can remain as a complementary option.

### Alternatives considered

- **"Ask Question" side chat** (new Agents window): works, but spawns a separate side chat instead of adding the text to the current conversation input.
- **`#session` mention / drag a whole chat as context**: references an entire conversation, not a specific paragraph.
- **Copy & paste**: works, but is exactly the manual workaround this request aims to eliminate.

### Additional context

- VS Code stable, latest (2026). Copilot Chat is now bundled in this repo (`extensions/copilot`).
- This mirrors the well-received "Add to conversation" selection UX from OpenAI Codex, which many users expect in Copilot Chat.
- No public extension API can implement this today (chat response selection is not exposed), so it needs to be a built-in feature.