This removes the `gemini-and-native` and `claude-code` feature flags.
Also, I removed a bunch of unused agent1 code that we do not need
anymore.

Initially I wanted to remove much more of the `agent` code, but noticed
some things that we need to figure out first:
- The inline assistant + context strip use `Thread`/`ContextStore`
directly
- We need some replacement for `ToolWorkingSet`, so we can access
available tools (as well as context server tools) in other places, e.g.
the agent configuration and the configure profile modal

Release Notes:

- N/A