fix(rules-core): fall back to workspace directory when no project root marker is found

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

src/hooks/rules-injector/injector.test.ts

@@ -339,6 +339,7 @@ describe("createRuleInjectionProcessor", () => {
 				contentHashes: new Set<string>(),
 				realPaths: new Set<string>([ruleRealPath]),
 			}),
+			homedir: () => homeRoot,
 		});
 
 		// when