[VSCode] Regression: current-file chip can no longer be persistently hidden (Hide toggle replaced by X)

## Summary

Version 2.1.261 changed the current-file chip in the message box: the **Hide** toggle was replaced by an **X** that removes the chip. The changelog lists this as a UI tweak, but it removed a feature. There is now no way to keep the active editor file out of the prompt context. The chip comes back every time I switch files, so I have to click X again before almost every message.

> Changelog 2.1.261: *"[VSCode] Changed the current-file chip in the message box: an X now removes it, replacing the Hide toggle"*

## Previous behavior (before 2.1.261)

- Clicking the chip toggled it to hidden.
- It stayed hidden while I switched between editor tabs, so I could look through several files without them being attached to my messages.

## Current behavior (2.1.261+)

- Clicking the X removes the chip, but only until I switch tabs or open another file.
- As soon as I open another file (which happens constantly while reviewing code), the chip comes back and that file is attached to the next message again.
- No `claudeCode.*` VS Code setting and no `settings.json` key turns this off.

## Why this matters

A common workflow is to read the code in the editor, jumping across many files, while chatting with Claude about something that has nothing to do with whichever file happens to be open. In that workflow:

- The file attached automatically is usually unrelated to the question. It adds noise to the context and can point the model at the wrong file.
- I have to remember to click X before every message. If I forget, the wrong file gets attached.
- The old toggle handled this well. The new design turns a one-time choice into something I have to repeat all the time.

## Steps to reproduce

1. Open any file in VS Code with the Claude Code extension (2.1.261 or later).
2. Click X on the current-file chip in the message box.
3. Open or switch to another file.
4. The chip is back, and the new file will be attached to the next message.

## Requested fix (any of these would work)

1. **Bring back the persistent Hide toggle**, either instead of the X or next to it, or
2. **Add a setting** such as `claudeCode.autoAttachActiveFile: false` that turns off automatic current-file context, or
3. **Make the X persistent** for the session, so the chip stays removed until I turn it back on.

## Environment

- Claude Code VS Code extension: 2.1.261+
- OS: Windows 10

Reproduce the behavior in the VS Code extension by opening a file, removing the current-file chip with X, and switching files. Trace the current-file chip and its persistence behavior; done means the user can keep automatic file context hidden across file switches, either through a persistent Hide action, a setting, or session-persistent removal.

索引モデルが issue の本文から書いたものです。