Add AGENTS.md contributor guide and development/testing conventions

[ggrell]

Motivation

• Add a newcomer and contributor guide to centralize repository overview, development workflow, and testing/style expectations for the RxReactor project.

Description

• Add AGENTS.md documenting repository structure, core concepts like Reactor/ReactorWithEffects/SimpleReactor, and the recommended learning path.
• Document development and code-style expectations including Detekt rules and Kotlin idioms, and include a BSD-3-Clause file header template for new files.
• Provide explicit unit test guidance covering frameworks (JUnit 4), RxJava TestObserver, assertion preferences, test placement across rxreactor1/rxreactor2/rxreactor3, and when to run ./gradlew check.
• Include CI/workflow references and instructions for running targeted module tests during development.

Testing

• No automated tests were added or modified as part of this documentation-only change.
• Recommend running ./gradlew check locally and relying on existing CI workflows to validate formatting, static analysis, and tests.

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Codex Task

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.11%. Comparing base (a04b900) to head (b40e596).
⚠️ Report is 14 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##               main     #306   +/-   ##
=========================================
  Coverage     88.11%   88.11%           
  Complexity       48       48           
=========================================
  Files             9        9           
  Lines           101      101           
=========================================
  Hits             89       89           
  Misses           12       12           

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