fix: credential lifecycle safety -- update_item, duplicate prevention, tests

CRITICAL FIX: The --enable-2fa flow was creating duplicate BW items
instead of updating in place, which led to ambiguous item resolution
and data integrity issues. This was a severe failure in core
credential lifecycle operations.

Changes:
- bw_helper.py: Complete rewrite with safety guarantees
  - update_item(): modifies existing item in place by ID, preserves
    all fields not being updated
  - create_item(): refuses to create duplicates (raises if item exists)
  - get_item_id(): resolves name to ID, raises on ambiguous matches
  - get_item(): returns full item JSON
  - NO delete_item method exists by design -- credential deletion
    is a manual operation only
- provision-agent.py: --enable-2fa now uses update_item() instead
  of create_item() to add TOTP to existing credentials
- Dockerfile: non-root user with correct BW state directory ownership
- docker-compose.yml: bind mount for BW state (proper permissions)
- test_bw_helper.py: 10 tests covering full lifecycle
  (create, read, duplicate rejection, update password, update TOTP,
  field preservation, no-delete verification)
  Tests 1-4 verified passing against live Vaultwarden instance.
- requirements.txt: added pytest

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Assisted-by: Crush:glm-5.2

WORKING.md

@@ -1,11 +1,13 @@
-# WORKING.md
+# WORKING.md — Active Session Tracker
 
-- [x] Create Gitea repo
-- [x] Adopt TSYSGroupAIOS framework
-- [x] Build Dockerfile + docker-compose.yml
-- [x] Build manifest template (agents.yaml.example)
-- [x] Write provision-agent.py
-- [x] Write bw-helper.py
-- [x] Install git hooks
-- [x] Shellcheck on all scripts
-- [x] Commit + push
+Agent work only. User actions (deploy, review, UAT) are NOT tracked here.
+The human decides when the work is "done".
+
+## Current Tasks
+
+- [ ] Add update_item() to bw_helper.py (root cause of credential deletion)
+- [ ] Fix --enable-2fa to update item in place, never create duplicates
+- [ ] Add duplicate-prevention safeguard to create_item()
+- [ ] Write credential lifecycle tests (create, read, update, never delete)
+- [ ] Verify 2FA is enabled and working end-to-end
+- [ ] Update STATUS.md