Fix citation validator missing backslash-rooted paths on POSIX

The absolute-path rejection relied on path.isAbsolute(), which is
platform-dependent: on Linux, backslash-rooted paths like \src\a.ts are
not considered absolute, so Windows-generated artifacts with such evidence
paths slipped past validation when the server runs on POSIX. Drive
prefixes (C:/) were already covered by DRIVE_PREFIX; this adds the
explicit leading-backslash check.

Caught by CI on ubuntu (tests pass on Windows); verified on both Windows
and Linux Node 22 before pushing.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: HelloThisWorld

src/validation/citation-validator.ts

@@ -37,7 +37,7 @@ export function validateEvidenceShape(evidence: Evidence, location: string): Cit
 
   if (file.trim().length === 0) {
     issues.push({ location, message: "evidence file path must not be empty" });
-  } else if (isAbsolute(file) || DRIVE_PREFIX.test(file) || file.startsWith("/")) {
+  } else if (isAbsolute(file) || DRIVE_PREFIX.test(file) || file.startsWith("/") || file.startsWith("\\")) {
     issues.push({
       location,
       message: `evidence file path must be relative to the repository root, got "${file}"`,