Bug/Feat: Sandbox config to selectively enable tools

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

Feature Request: Sandbox config in settings.json to selectively enable tools, and/or whitelist bundled copilot package tools

### Proposed solution

potentially have some tools { ... } within sandbox section in settings.json to enable allowed tools/apps/executables
_(or provide documentation on how best to accomplish this if fits somehow in current schema)_

more specifically, encountered this around copilot bundled rg ripgrep, while in sandbox enabled:
```
Search "xunit\.V3" (sandboxed) sh: 1:~/.copilot/pkg/linux-x64/1.0.75/ripgrep/bin/linux-x64/rg: not found

Search is blocked because the bundled  rg  executable is unavailable under the current sandbox policy.
Please review  /sandbox  and retry.
```

ideally specifically around this, executable packages that are bundled within distributed copilot are just marked "safe" by default and end user does not need to do anything additional to whitelist to allow these in sandbox mode

### Example prompts or workflows

_No response_

### Additional context

_No response_

Start by reading the settings.json configuration and the /sandbox policy behavior, then trace how the bundled executable at ~/.copilot/pkg/linux-x64/1.0.75/ripgrep/bin/linux-x64/rg is checked. Determine whether the existing schema can express allowed tools or whether bundled tools need a safe default; done means the intended configuration or documentation is defined and sandboxed Search can use rg.

Written by the indexing model from the issue text.