Rewrite AGENTS.md in second person

[forsyth2]

Summary

Objectives:

• Rewrite AGENTS.md in second person, with an audience of agents in mind.
• Rewrite instructions and context to be clearer.

Issue resolution:

• Closes [Doc]: Update AGENTS.md for better Copilot functionality #794
• Help AI agents address the prompt in the comments of Review livvkit #790.

Select one: This pull request is...

• a bug fix: increment the patch version
• a small improvement: increment the minor version
• a new feature: increment the minor version
• an incompatible (non-backwards compatible) API change: increment the major version

Small Change

• To merge, I will use "Squash and merge". That is, this change should be a single commit.
• Logic: I have visually inspected the entire pull request myself.
• Pre-commit checks: All the pre-commits checks have passed.

[forsyth2]

Based on E3SM-Project/E3SM#8169 (comment), we'll also need to add a .github/copilot-instructions.md file.