Update LICENSE copyright and add AI attribution instructions

[jerrysxie]

This PR applies two org-wide standardisation changes
(ref: OpenDevicePartnership/embedded-rust-template#20):

1. LICENSE – normalise the copyright line to:
   Copyright (c) Open Device Partnership and Contributors

2. .github/copilot-instructions.md – add the Commit Messages and
   AI Attribution sections so every repo carries consistent guidance
   for contributors and AI agents.

[Copilot]

Pull request overview

This PR applies org-wide standardization by updating the MIT license copyright attribution and adding consistent contributor/AI-agent guidance in the repository’s Copilot instructions.

Changes:

• Normalize the MIT LICENSE copyright line to “Open Device Partnership and Contributors”.
• Add commit message formatting guidance to .github/copilot-instructions.md.
• Add a standardized “AI Attribution” policy describing an Assisted-by commit trailer and prohibiting AI agents from adding Signed-off-by.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
LICENSE	Updates the copyright attribution line to the org-standard wording.
.github/copilot-instructions.md	Adds standardized commit message and AI attribution guidance for contributors and AI agents.