refactor(code-executor): migrate user scripts to single context argument

- replace (page, context, helpers) signature with async (context) => {}
- add ScriptContext interface exposing page, browser, env, getEnv,
  getCredential, getStepOutput, runSnippet, dumpDom, log/warn/error
- rename getEnvUrl to getEnv throughout service and docs
- fix step ordering: normalizeStepOrder and run step rows are now 1-based
- migrate existing DB rows (scenario_steps, scenario_run_steps) +1
- update all tests and stored snippet/step code in DB to new API

docs/scenario.md

@@ -33,7 +33,7 @@ Current scheduler behavior is exec-centric:
 - `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.getEnvUrl(key)`: required environment URL lookup (throws if missing)
+- `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
 
 ## Validation contract