Task: Create a Python helper module named tools.py that exposes simple functions for our Android to use with Supabase.

Environment:
- Use env vars SUPABASE_URL and SUPABASE_SERVICE_KEY.
- Use supabase-py v2 (already installed).

Create tools.py with the following:

1) get_client()
   - Returns a Supabase client using env vars.
   - Raises a clear error if env vars are missing.

2) resolve_agreement_id(agreement_number: str) -> int | None
   - Looks up agreement.id by agreement_number from the "agreement" table.
   - Returns None if not found.

3) log_message(agreement_number: str, channel: str, direction: str, text: str) -> dict
   - Inserts into contact_history:
     fields: agreement_id, channel, direction, message_text
   - Uses resolve_agreement_id first.
   - Returns the inserted row (id, agreement_id, channel, direction, happened_at).

4) open_thread(agreement_number: str, thread_type: str = 'renewal') -> dict
   - Upserts into open_thread with fields:
     agreement_number, thread_type, status='open'
   - If a row exists with status='open', return it; otherwise create one.
   - Return the row (id, agreement_number, status, created_at).

5) close_thread(agreement_number: str) -> dict | None
   - Finds the most recent open_thread for this agreement_number with status='open'.
   - Sets status='closed', updated_at=now().
   - Returns the updated row or None if none open.

6) schedule_followup(agreement_number: str, due_at_iso: str, reason: str) -> dict
   - Inserts into follow_up:
     fields: agreement_id, due_at (ISO string), reason
   - Returns the inserted row (id, agreement_id, due_at, reason).

7) book_appt_request(agreement_number: str, preferred_date: str | None = None, preferred_time: str | None = None, notes: str | None = None, channel: str = 'whatsapp', requested_by: str = 'ai') -> dict
   - Inserts into appointment_request:
     fields: agreement_id, requested_by, channel, preferred_date, preferred_time, notes
   - Returns inserted row (id, agreement_id, status, created_at).

8) get_equity_snapshot(agreement_number: str) -> dict | None
   - Resolve agreement_id, then select from view v_equity_latest where agreement_id = <id>.
   - Return {agreement_id, snapshot_date, settlement, balloon, true_equity, equity, parity} or None.

Implementation notes:
- Use concise, defensive error handling: raise ValueError with a helpful message if agreement_number not found.
- Keep prints out of the module; return dicts.
- Add minimal docstrings and type hints.
- Do not modify other files.

Deliverable:
- Create/overwrite tools.py at the project root.
- When done, print: CREATED tools.py