Adopt shared-SA model, add DEPLOY guide, MIT license

- Install flow now assumes one admin-distributed service account JSON shared
  across the team. Each user creates their own Google Sheet and shares it
  with the SA email; per-user GCP project setup is gone.
- DEPLOY.md: a step-by-step walkthrough for non-technical users following
  along with an AI agent (Terminal basics, screenshots-worth of expected
  output, common-error table).
- scripts/append_row.py: sheet_id field accepts either a bare ID or a full
  Google Sheets URL; normalize_sheet_id() extracts the ID via regex.
- scripts/setup.md: rewritten as an admin guide (one-time GCP setup, key
  rotation) plus a troubleshooting reference.
- LICENSE: MIT (previously "private — internal use"). README license
  sections updated to match.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

config.example.json

@@ -1,6 +1,6 @@
 {
-  "sheet_id": "PUT_GOOGLE_SHEET_ID_HERE",
-  "worksheet": "Expenses",
+  "sheet_id": "PASTE_YOUR_GOOGLE_SHEET_URL_OR_ID_HERE",
+  "worksheet": "Sheet1",
   "service_account_path": "~/.config/gcp/autoacct-sa.json",
   "hkd_fx_provider": "frankfurter"
 }