Please read and understand GitHub issue #37: "Persistent config for Claude credentials mounting".

Issue description:
Currently `--claude-credentials` must be passed on every `bubble open` invocation. There's no way to make it the default.

## Proposed changes

### Config setting

Add a `[claude]` section to `~/.bubble/config.toml`:

```toml
[claude]
credentials = true   # default: false
```

When set, `--claude-credentials` behavior is enabled by default. `--no-claude-credentials` on the CLI would override it.

### CLI command

Add a subcommand for toggling this:

```bash
bubble claude credentials on    # sets [claude] credentials = true
bubble claude credentials off   # sets [claude] credentials = false
bubble claude status            # shows current settings
```

This would also suppress the nag message ("Tip: use --claude-credentials...") when credentials are explicitly disabled via `bubble claude credentials off`, since the user has made a deliberate choice.

🤖 Prepared with Claude Code



Please claim this issue (assign it to yourself if possible), then implement a fix or feature as described. Work on a branch named `issue-37`, and open a PR when done.