feat(btw): add experimental.btw setting and documentation

- Added `experimental.btw` setting to the settings schema.
- Updated `/btw` command to check if `experimental.btw` is enabled.
- Added documentation for `/btw` in `docs/reference/commands.md`.
- Regenerated settings documentation in `docs/reference/configuration.md` and `docs/cli/settings.md`.
- Updated unit tests for `/btw` command to provide mock context and test enablement flag.

docs/reference/commands.md

@@ -49,6 +49,13 @@ Slash commands provide meta-level control over the CLI itself.
   behavior can be modified using the `advanced.bugCommand` setting in your
   `.gemini/settings.json` files.
 
+### `/btw`
+
+- **Description:** Ask a side question without affecting history (ephemeral).
+- **Note:** This command is experimental and requires `experimental.btw: true`
+  in your `settings.json`.
+- **Usage:** `/btw <question>`
+
 ### `/chat`
 
 - **Description:** Alias for `/resume`. Both commands now expose the same

docs/reference/configuration.md

@@ -1725,6 +1725,11 @@ their corresponding top-level category object in your `settings.json` file.
     for reduced chattiness and structured progress reporting.
   - **Default:** `false`
 
+- **`experimental.btw`** (boolean):
+  - **Description:** Enable the experimental /btw side inquiry command for
+    ephemeral, non-persisted chat turns.
+  - **Default:** `false`
+
 #### `skills`
 
 - **`skills.enabled`** (boolean):