Improve repo-assist workflow and documentation - #380

Summary

• fold the general Repo Assist workflow improvements back into the shared workflow
• add strict schema validation for persistent Repo Assist memory
• refine task selection, fallback behavior, backlog tracking, and monthly activity reporting
• update the Repo Assist documentation to match

Release note

These changes require a new gh-aw release before they can be used.

Validation

Not run, per request; validation will be completed later.