feat(feed): make episode cache bound user-configurable (date window or count)

Add episodeCacheMode/count/days preferences (default: date, 60 days).
Apply the bound when reading instead of writing, so a preference change
takes effect without a refetch; the full parse cache stays intact so
fetch-more can page beyond the bound. Thread the window through
load/saveFeedsToFile and update tests and task docs.

.gitignore

@@ -35,3 +35,5 @@ report.[0-9]*.[0-9]*.[0-9]*.[0-9]*.json
 .harness/
 .ralpi
 notes.md
+# pygienium run-state and check artifacts
+.pygienium/