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/add_github_team_members.py

@@ -1,15 +1,29 @@
 #!/usr/bin/env python3
-"""Add GitHub users to their appropriate teams in nhcarrigan-spring-2026-cohort org"""
+"""Add GitHub users to their appropriate teams in the cohort GitHub organisation.
+
+Uses the GitHub CLI to add each member to their corresponding team and, for
+leaders, to the team's leaders sub-team.
+
+Data files (place in data/):
+  - team_assignments.json   Team rosters with leaders and participants per team
+  - discord_to_github.json  Mapping of Discord IDs to GitHub usernames
+
+Env vars:
+  - None (uses `gh` CLI for authentication)
+"""
 
 import json
 import subprocess
 import time
+from pathlib import Path
+
+DATA_DIR = Path(__file__).parent.parent.parent / "data"
 
 # Load team assignments and Discord to GitHub mappings
-with open("team_assignments.json") as f:
+with open(DATA_DIR / "team_assignments.json") as f:
     teams = json.load(f)
 
-with open("discord_to_github.json") as f:
+with open(DATA_DIR / "discord_to_github.json") as f:
     discord_to_github = json.load(f)
 
 # Map team names to GitHub team slugs