Add NOTES.md documenting implementation choices

- Plan: PUT endpoint with validation and 404 handling
- Model: Claude Opus for pattern consistency
- Commits: Split between data access and routing layers
- Review: Correct implementation on first pass, all tests pass

Author: pawunder1992

NOTES.md

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+# Implementation Notes
+
+## Plan
+The approved plan outlined adding a PUT `/users/:id` endpoint to update existing users, with input validation (rejecting missing `name` or `email` fields with 400 status), proper 404 handling when users don't exist, and data access through the existing `db/store.js` abstraction. I did not edit the plan before approving it—it accurately captured the requirements from the test file and matched the existing patterns in the codebase.
+
+## Model Choice
+I used Claude Opus (Combo model configured as default for this session). This was appropriate for the task because it provides strong reasoning for understanding existing patterns and architectural consistency, which was key to maintaining the codebase's clean abstraction layers between routes and data access.
+
+## Commit Strategy
+I have not yet made commits for this work—the implementation was completed as a planning and verification phase. When commits are made, they should be split as: (1) one commit for the `updateUser` function in `db/store.js` with its export, and (2) a second commit for the PUT route handler in `routes/users.js`. This split keeps data access logic separate from HTTP routing logic, matching the existing file structure and making it easier to review and revert individual concerns if needed.
+
+## Review Findings
+The implementation required no corrections—it was correct on first pass. All three test cases passed without modification: successful updates return 200 with updated fields, non-existent users return 404, and missing required fields return 400. The code follows the existing patterns precisely: mirroring the POST endpoint's validation structure, using the same error message format, and implementing the store function with the same simple, side-effect-on-array pattern as existing functions.