fix: load all agent rules files (AGENTS.md, AGENT.md, AGENTS.local.md) when present

- Remove break statement to load both AGENTS.md and AGENT.md if they exist
- Update function documentation to clarify all files are loaded
- Update test to verify both files are loaded when both exist

Author: roomote

src/core/prompts/sections/__tests__/custom-instructions.spec.ts

@@ -862,7 +862,7 @@ describe("addCustomInstructions", () => {
 		expect(readFileMock).toHaveBeenCalledWith(expect.stringContaining("AGENT.md"), "utf-8")
 	})
 
-	it("should prefer AGENTS.md over AGENT.md when both exist", async () => {
+	it("should load both AGENTS.md and AGENT.md when both exist", async () => {
 		// Simulate no .roo/rules-test-mode directory
 		statMock.mockRejectedValueOnce({ code: "ENOENT" })
 
@@ -902,11 +902,13 @@ describe("addCustomInstructions", () => {
 			},
 		)
 
-		// Should contain AGENTS.md content (preferred) and not AGENT.md
+		// Should contain BOTH AGENTS.md and AGENT.md content
 		expect(result).toContain("# Agent Rules Standard (AGENTS.md):")
 		expect(result).toContain("Agent rules from AGENTS.md file (plural)")
-		expect(result).not.toContain("Agent rules from AGENT.md file (singular)")
+		expect(result).toContain("# Agent Rules Standard (AGENT.md):")
+		expect(result).toContain("Agent rules from AGENT.md file (singular)")
 		expect(readFileMock).toHaveBeenCalledWith(expect.stringContaining("AGENTS.md"), "utf-8")
+		expect(readFileMock).toHaveBeenCalledWith(expect.stringContaining("AGENT.md"), "utf-8")
 	})
 
 	it("should return empty string when no instructions provided", async () => {

src/core/prompts/sections/custom-instructions.ts

@@ -276,12 +276,13 @@ async function readAgentRulesFile(filePath: string): Promise<string> {
 }
 
 /**
- * Load AGENTS.md or AGENT.md file from a specific directory
- * Checks for both AGENTS.md (standard) and AGENT.md (alternative) for compatibility
- * Also loads AGENTS.local.md for personal overrides (not checked in to version control)
- * AGENTS.local.md can be loaded even if AGENTS.md doesn't exist
+ * Load agent rules files from a specific directory
+ * Loads ALL of the following files if they exist:
+ * - AGENTS.md (standard agent rules)
+ * - AGENT.md (alternative naming)
+ * - AGENTS.local.md (personal overrides, not checked in to version control)
  *
- * @param directory - Directory to check for AGENTS.md
+ * @param directory - Directory to check for agent rules files
  * @param showPath - Whether to include the directory path in the header
  * @param cwd - Current working directory for computing relative paths (optional)
  */
@@ -306,9 +307,7 @@ async function loadAgentRulesFileFromDirectory(
 					? `# Agent Rules Standard (${filename}) from ${displayPath}:`
 					: `# Agent Rules Standard (${filename}):`
 				results.push(`${header}\n${content}`)
-
-				// Found a standard file, don't check alternative
-				break
+				// Continue to check for additional agent rules files
 			}
 		} catch (err) {
 			// Silently ignore errors - agent rules files are optional