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/scenario.md

@@ -17,24 +17,28 @@ Current scheduler behavior is exec-centric:
 
 ## Script runtime
 
-`execCode` and `validateCode` execute as async JavaScript with:
+`execCode` and `validateCode` execute as async JavaScript with a single `context` argument:
 
-- `page`: Playwright `Page`
-- `context`: Playwright `BrowserContext`
-- `helpers`: utility object
+```js
+async (context) => {
+  // your code here
+}
+```
+
+Available in scope:
+
+- `context.page`: Playwright `Page`
+- `context.browser`: Playwright `BrowserContext`
+- `context.env`: shallow copy of environment key/value map
+- `context.getEnv(key)`: required environment value lookup (throws if missing)
+- `context.getCredential(alias)`: credential payload assigned to the scenario alias
+- `context.getStepOutput(order)`: prior step output by absolute order (`1`, `2`, ...) or relative (`-1` = previous step)
+- `context.dumpDom(selector?)`: simplified DOM snapshot
+- `context.log(...args)`, `context.warn(...args)`, `context.error(...args)`: structured step logs
+- `context.runSnippet(name, ...args)`: execute stored snippet code with the same context
 - `console`: proxied to run logs (`log`, `warn`, `error`, etc.)
 
-`validateCode` additionally receives `result`, which is the value returned by `execCode`.
-
-## Available helpers
-
-- `helpers.dumpDom(selector?)`: simplified DOM snapshot
-- `helpers.log(...args)`, `helpers.warn(...args)`, `helpers.error(...args)`: structured step logs
-- `helpers.getStepOutput(order)`: prior step output by absolute order (`0`, `1`, ...) or relative (`-1` previous step)
-- `helpers.getCredential(alias)`: credential payload assigned to the scenario alias
-- `helpers.env`: shallow copy of environment URL map
-- `helpers.getEnv(key)`: required environment value lookup (throws if missing)
-- `helpers.runSnippet(name, ...args)`: execute stored snippet code in the same page/context/helpers scope
+`validateCode` additionally receives `result` in scope, which is the value returned by `execCode`.
 
 ## Validation contract