Option to "wipe" stored memories (<repository_memories>)

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

Option to clear or disable "server-side" ``

### Proposed solution

/wipe or something like that to wipe "stored_memories".

### Example prompts or workflows

https://gist.github.com/dnoriegagoodwin/4ddaeb235ab58db8c1114abef50ca4d8

### Additional context

I noticed a session kept saying "memory stored" or something like that. When I asked, CCLI said:

```
● Check config for memory settings
$ # Memories are server-side, but let's check if there's any local trace
grep -r "repository_memories\|store_memory" ~/.copilot/config.json 2>/dev/null; echo "---"; cat ~/.copilot/config.json 2>/dev/null | python3 -m json.tool 2>/dev/null | head -30

● They're stored server-side by GitHub — tied to your account + repository. The store_memory tool sends facts to GitHub's backend, and they're injected into the block in your system prompt
each session. No local file to delete.

To clear existing memories, I'm not aware of a CLI command or config option for that. Your best bet is /feedback to request a memory purge, or filing on the Copilot CLI repo. The instruction we added will at least stop new ones from accumulating.
```

Full context where I interrogated can be found here.

I interrogated when I saw it referred to me as "Brady" (not my name).

https://gist.github.com/dnoriegagoodwin/4ddaeb235ab58db8c1114abef50ca4d8#-copilot-8