feat(browser,mcp): resolve environment and credentials from DB in exec

- POST /exec now accepts environmentId (UUID) and credentials (alias → UUID)
  instead of raw payloads; resolves entities via EnvironmentService and
  CredentialService before passing data to browserService.exec
- exec_code MCP tool updated with same schema: environmentId + credentials map
- CredentialModule imported into BrowserModule and McpModule
- docs(scenario): rewrite script runtime section for single context argument
- docs(mcp): update exec_code description to reflect new parameter shapes
- style: reorder imports across server source (formatter)

docs/mcp.md

@@ -28,7 +28,7 @@ The server currently registers the following tools.
 | Tool | Description |
 |---|---|
 | `open_url` | Open URL with optional `sessionName`, return page/title/content |
-| `exec_code` | Execute Playwright JS with `page` and `context` in scope |
+| `exec_code` | Execute Playwright JS — script receives a single `context` argument (see `scenario.md`). Optional `environmentId` (UUID) resolves environment from DB for `context.getEnv()`. Optional `credentials` (`{ alias: credentialUUID }`) resolves credentials from DB for `context.getCredential(alias)`. |
 
 ## Scenarios