#334 More custom code cleanup

Summary by cubic

Standardized custom-code boundaries in the client and tightened typing for session resources. Hardened the example by validating observe results as dicts and restored the _sea keep file.

• Refactors

  • Standardized custom-code markers across Stagehand and AsyncStagehand methods; no logic changes.
  • Clarified sessions accessors’ type hints via TYPE_CHECKING imports; runtime imports unchanged.
  • Updated with_options comment (start → create).
  • Restored src/stagehand/_sea/.keep to keep the directory in VCS.

• Bug Fixes

  • Example: Validate observe stream items are dicts and read ['description']; raises a clear error if not.