feat: Enable environment variable substitution in settings

This commit introduces the ability to use system environment variables
within the settings files (e.g., `settings.json`). Users can now
reference environment variables using the `${VAR_NAME}` syntax.

This enhancement improves security and flexibility, particularly
for configurations like MCP server settings, which often require
sensitive tokens.

Previously, to configure an MCP server, a token might be directly
embedded:
```json
"mcpServers": {
  "github": {
    "env": {
      "GITHUB_PERSONAL_ACCESS_TOKEN": "pat_abc123"
    }
    // ...
  }
}
```

With this change, the same configuration can securely reference an
environment variable:
```json
"mcpServers": {
  "github": {
    "env": {
      "GITHUB_PERSONAL_ACCESS_TOKEN": "${GITHUB_PERSONAL_ACCESS_TOKEN}"
    }
    // ...
  }
}
```

This allows users to avoid storing secrets directly in configuration files.

0 files changed, 0 insertions, 0 deletions