docs: add data file documentation and fix data path resolution

All Python cohort scripts now use DATA_DIR = Path(__file__).parent.parent.parent / "data"
to correctly resolve the repo-root data/ directory regardless of the working directory
set by run.sh. All TypeScript scripts have expanded JSDoc headers documenting data file
requirements and environment variables.

python/cohort/check_lengths.py

@@ -1,4 +1,15 @@
-"""Quick dry-run to check Discord message lengths before sending."""
+"""Dry-run check of Discord message lengths before sending the activity report.
+
+Parses the catch_up_report.md table, formats each team's data into a monospace
+Discord table, and reports whether any message would exceed Discord's 2000-char limit.
+Run this before send_activity_report.py to catch length issues early.
+
+Data files (place in data/):
+  - catch_up_report.md  Activity report generated by catch_up_report.py
+
+Env vars:
+  - None
+"""
 
 FIELDS = [
     ("Discord Username", "Name", 18),
@@ -11,7 +22,7 @@ FIELDS = [
     ("Commits", "Commits", 7),
 ]
 
-REPORT_PATH = "catch_up_report.md"
+REPORT_PATH = "data/catch_up_report.md"
 
 
 def parse_report(path: str) -> dict[str, list[dict]]: