docs: replace specific agent file with general repo-wide AGENTS.md style rules

Author: lahirumaramba

AGENT.md

@@ -0,0 +1,33 @@
+# AI coding assistant guidelines: Firebase Admin Java SDK
+
+This document defines repository-wide expectations, code styles, and Checkstyle compliance metrics that every autonomous AI coding agent or pair programming assistant must strictly follow when introducing or modifying code in this repository.
+
+---
+
+## 🎨 Code Style & Checkstyle Compliance
+
+This repository enforces standard **Google Java Style** guidelines validated through automated Maven checkstyle executions (`checkstyle.xml`). Any compilation or pull request build will fail if these constraints are violated.
+
+### 1. Strict 100-Character Line Limit
+*   **Rule:** No line of code, comments, Javadoc documentation entries, or inline string literal concatenations may exceed **100 characters** under any circumstances.
+*   **Remediation:** 
+    *   Break long method parameters, array initializations, and logic comparisons onto separate lines.
+    *   Wrap long `assertThrows` statements or lambda chains across multiple contiguous lines.
+    *   Format long log messages, exception messages, or assertion string explanations using standard string block concatenation broken across separate lines.
+
+### 2. Import Ordering & Grouping
+*   **Rule:** Imports must be grouped and arranged alphabetically to avoid validation noise.
+*   **Remediation:** 
+    1.  Static imports placed first, grouped, and alphabetically arranged.
+    2.  Non-static imports grouped alphabetically by package tier.
+    3.  Avoid utilizing wildcard (`*`) imports. Every import declaration must be explicit.
+
+### 3. Javadoc Completeness & Formatting
+*   **Rule:** Every public class, package-private component interface, constructor, and public method signature must include fully formed Javadoc documentation blocks.
+*   **Remediation:** 
+    *   Document all arguments via `@param`, explain error flows via `@throws`, and clear return constraints via `@return`.
+    *   Wrap documentation description text explicitly so that no single javadoc documentation line passes the 100 characters limit.
+
+### 4. Indentation & Spacing
+*   **Rule:** Standard indentation uses exactly **2 spaces** per block indentation level. **4 spaces** are used explicitly for wrapped line continuation indentation.
+*   **Remediation:** Never utilize tab characters. Ensure block braces follow the Google K&R opening line placement convention.