extract mermaid docs

docs/pipeline/diagrams/planned/activity.md

@@ -0,0 +1,7 @@
+# The Biergarten Data Pipeline — Activity Diagram (Planned)
+
+Each generation phase runs a producer/consumer trio concurrently: an **Orchestrator** producer feeds a bounded channel, an **LLM Worker** consumes it and generates records (closing its output channel when the input is drained and closed), and a **SQLite Worker** consumes that output channel and commits in batches. The Checkin and Follow phases run in parallel with each other once the user and brewery pools are populated.
+
+```mermaid
+--8<-- "diagrams/pipeline/planned/activity.mmd"
+```