Adds a docs/solutions learning: a Task field is silently dropped on persist
unless it has a SQLite column + defineTaskColumn + rowToTask mapping (applyTaskPatch
writes the DB-round-tripped task back over task.json). Plus a CONCEPTS.md entry for
the active-session lease (the path-keyed exclusivity/liveness registry whose key
choice caused the concurrent-workspace collision).
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
