docs: enforce PR template usage

[tcheeric]

Why now?

AGENTS.md lacked explicit instruction to follow the PR template, leading to inconsistent submissions.
Related issue: #0

What changed?

• Added guidance to AGENTS.md to always follow PR submission guidelines and use the repository's PR template.

BREAKING

None.

Review focus

Ensure the new AGENTS.md instructions are clear and capture repository expectations.

Checklist

• Scope ≤ 300 lines (or split/stack)
• Title is verb + object (e.g., “Refactor auth middleware to async”)
• Description links the issue and answers “why now?”
• BREAKING flagged if needed
• Tests/docs updated (if relevant)

Testing

mvn -q verify

[ERROR] Tests run: 13, Failures: 4, Errors: 0, Skipped: 0
[ERROR] Failed to execute goal org.apache.maven.plugins:maven-surefire-plugin:2.22.2:test (default-test) on project phoenixd-model: There are test failures.
[ERROR] Please refer to /workspace/phoenixd-java/phoenixd-model/target/surefire-reports for the individual test results.
[ERROR] -> [Help 1]
[ERROR] To see the full stack trace of the errors, re-run Maven with the -e switch.
[ERROR] Re-run Maven using the -X switch to enable full debug logging.
[ERROR] For more information about the errors and possible solutions, please read the following articles:
[ERROR] [Help 1] http://cwiki.apache.org/confluence/display/MAVEN/MojoFailureException
[ERROR] After correcting the problems, you can resume the build with the command
[ERROR]   mvn <args> -rf :phoenixd-model

Network Access

No external network requests were made.

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https://chatgpt.com/codex/tasks/task_b_68a6504329cc8331b794a6ffeea0d6cf