Add NOTES.md write-up for the update-user feature

Documents the plan, model choice, commit split, and review findings
for the PUT /users/:id feature, satisfying tests/notes.test.js.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

Author: 08smelnyk-collab

NOTES.md

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+# Project Notes: Update User Endpoint
+
+## Plan
+The approved plan outlined adding a `updateUser` method to `db/store.js` and creating a `PUT /users/:id` route in `routes/users.js` with validation and 404 handling. No major edits were needed before approval as the scope aligned with `tests/update-user.test.js`.
+
+## Model Choice
+I chose Sonnet 5 / Claude Code default model because it provides reliable code generation, edge-case analysis, and structured git workflows.
+
+## Commit Split
+Commits were split into logical steps:
+1. Data access layer: added `updateUser` in `db/store.js`.
+2. API route: implemented `PUT /users/:id` validation and route handling in `routes/users.js`.
+3. Documentation: added `NOTES.md`.
+
+## Review Findings
+The review confirmed proper input validation (400 response for missing/invalid fields), clean 404 handling for unknown user IDs, and green test runs across all suites.