Complete credential item CRUD endpoints

[hamza-mohd]

Summary

• add authenticated /api/credentials/{id} read, update, and delete handlers
• validate credential item mutations and return item-level responses
• extend tests and docs for the full credential item surface

Notes

• This is PR 2 of an 8-PR stacked series.
• Base branch: feat/credentials-foundation.

[Copilot]

Pull request overview

Adds the missing authenticated item-level CRUD endpoints for credentials (/api/credentials/{id}) to complete the credential surface introduced by the collection endpoints, along with tests and route/docs updates.

Changes:

• Register GET, PUT, and DELETE routes for /api/credentials/{id} and include the route in the index route list.
• Implement credential store get/update/delete, request model for updates, and an item handler with validation.
• Add end-to-end server tests covering create → get → update → delete → 404-after-delete for a credential item.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
README.md	Documents the new credential item endpoints.
internal/provider/server.go	Registers item routes and exposes the item route in the index route list.
internal/provider/server_test.go	Adds coverage for credential item CRUD behavior.
internal/provider/credentials.go	Implements credential item handler and store operations plus update validation/model.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.