regression in 1.134.0: cannot search old chat sessions via `workbench.action.quickOpen`

Does this issue occur when all extensions are disabled?: Yes/No

- VS Code Version: Version: 1.134.0
- OS Version: MacOS 15.6.1 (24G90)

Steps to Reproduce:

1. Prior to Version: 1.134.0 we were able to browse thru recent agent chat sessions by typing `agent ` into `workbench.action.quickOpen` field. After the update however, typing `agent ` only searches inside repo, exposing only matching files, and no chats

Would be great to have this reverted. Or maybe document another way to quick browse / search for past agent chat sessions?

Before it looked smth like this (but ALL sessions were displayed, not just currently open ones):

Start by reproducing the regression in VS Code 1.134.0 through the workbench.action.quickOpen command, comparing searches for `agent ` with behavior before 1.134.0. Trace the quick-open handling for agent chat sessions and verify that searching exposes the expected past sessions rather than only matching repository files.

Written by the indexing model from the issue text.