docs(agents): guide contributors through repository workflows

[darion-yaphet]

What problem does this PR solve?

Issue Number: resolve N/A

Problem Summary:

The repository’s AGENTS.md only contained a minimal agent/security note. It did not provide practical contributor guidance for repository structure, build and test
workflows, coding style, testing expectations, PR conventions, or security/configuration reminders.

What is changed and the side effects?

Changed:

• Replaced the minimal agent note with a concise contributor guide tailored to the brpc repository.
• Documented key source, test, example, tool, documentation, packaging, and binding directories.
• Added common Make, CMake, Bazel, and example smoke-test commands.
• Summarized coding style, naming conventions, testing expectations, commit/PR guidance, and security tips.

Side effects:

• Performance effects: None. Documentation-only change.
• Breaking backward compatibility: None. No code or build behavior changed.

---

Check List:

• Please make sure your changes are compilable.
• When providing us with a new feature, it is best to add related tests.
• Please follow Contributor Covenant Code of Conduct.

Verification:

• Ran git diff --check -- AGENTS.md.
• Build and unit tests were not run because this is a documentation-only change.