update Markdown skills

skill-examples/board-invalid-context.md

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+---
+id: board-invalid-context
+name: Board (invalid context)
+description: Negative test — a position-scoped source on a placement that supplies no position.
+pages:
+  - positions
+status: active
+ui:
+  type: card
+  placement: after-position-list-summary
+  title: Board
+  source: position.activity
+  periods:
+    - today
+    - last-7-days
+    - previous-month
+---
+
+# Board (invalid context)
+
+## Purpose
+
+Deliberately wrong, and kept so the refusal stays tested. `position.activity`
+needs one position to read; `after-position-list-summary` renders once above the
+grid with no position in context. Saving this must be refused with a message that
+names both halves of the mismatch.