fix: add Monarch cookie bootstrap and saved-session auth

[mattebad]

Summary

• Add Monarch cookie bootstrap flow plus saved-session reuse so unattended runs can survive Monarch auth changes without interactive login.
• Update Monarch auth handling to prefer data/monarch_session.pickle, then cookie-string login, then the existing password/token compatibility paths.
• Refresh config, env examples, and README guidance so the new auth flow, split cookie vars, and pickle-only validation path are documented clearly.
• Add targeted tests for cookie parsing, login precedence, and the bootstrap command itself.

Testing

• Unit coverage:
  • python -m pytest tests/test_config.py tests/test_monarch_client_cache.py tests/test_cli_bootstrap_monarch_auth.py
  • Covers cookie env parsing, saved-session reuse, cookie bootstrap, stale-session recovery, and bootstrap session capture.
• Local live validation on macOS with Python 3.11:
  • python -m studentaid_monarch_sync bootstrap-monarch-auth
  • Manual browser login completed and data/monarch_session.pickle was saved successfully.
  • python -m studentaid_monarch_sync preflight
  • python -m studentaid_monarch_sync sync --dry-run
  • python -m studentaid_monarch_sync sync --dry-run --dry-run-check-monarch
• Pickle-only validation:
  • Confirmed the saved Monarch session works with cookie vars unset, which is the path intended for Docker/Unraid.

Notes

• The live validation used Python 3.11 because monarchmoneycommunity==1.4.0 requires Python >=3.10.
• Docker/Unraid usage is still supported via the saved session file and persistent data/ volume; no interactive login is needed on the box once the session pickle is bootstrapped.