fix(#1993): milestone --ws requirements archive header points at workstream path (#2015)

* fix(#1993): milestone --ws requirements archive header points at workstream path

The requirements archive header hardcoded the root .planning/REQUIREMENTS.md
path, so a workstream (--ws) archive pointed readers at the wrong file even
though #1917 fixed the archive LOCATIONS to land inside the workstream.

Derive the display path from the same workstream-aware reqPath the writer
already uses (path.relative(cwd, reqPath)). Root behavior is byte-identical
('.planning/REQUIREMENTS.md'); the --ws case now correctly reads
'.planning/workstreams/<ws>/REQUIREMENTS.md'.

- src/milestone.cts: reqDisplay interpolation in the archive header.
- tests/milestone.test.cjs: #1993 regression in the #1911 --ws block — header
  references the workstream path, not the root literal.

Closes #1993

* docs(#1993): backfill changeset pr 2015

* fix(#1993): use posix separators in archive header (Windows CI) + CRLF-safe test split

- src/milestone.cts: normalize path.relative output to POSIX separators so
  the workstream archive header renders forward slashes on Windows too
  (path.relative yields backslashes there; the original literal was posix).
- tests/milestone.test.cjs: .split(/\r?\n/) for the CRLF-fragile lint rule.

src/milestone.cts

@@ -284,7 +284,13 @@ function cmdMilestoneComplete(cwd: string, version: string, options: MilestoneCo
   // Archive REQUIREMENTS.md
   if (fs.existsSync(reqPath)) {
     const reqContent = fs.readFileSync(reqPath, 'utf-8');
-    const archiveHeader = `# Requirements Archive: ${version} ${milestoneName}\n\n**Archived:** ${today}\n**Status:** SHIPPED\n\nFor current requirements, see \`.planning/REQUIREMENTS.md\`.\n\n---\n\n`;
+    // Derive the display path from the same source the writer uses (reqPath), so a
+    // workstream archive header points at `.planning/workstreams/<ws>/REQUIREMENTS.md`
+    // instead of the hardcoded root path (#1993). Root case is byte-identical.
+    // Normalize to POSIX separators so the header is cross-platform (Windows
+    // path.relative yields backslashes; the original literal was forward-slash).
+    const reqDisplay = path.relative(cwd, reqPath).split(path.sep).join('/');
+    const archiveHeader = `# Requirements Archive: ${version} ${milestoneName}\n\n**Archived:** ${today}\n**Status:** SHIPPED\n\nFor current requirements, see \`${reqDisplay}\`.\n\n---\n\n`;
     platformWriteSync(path.join(archiveDir, `${version}-REQUIREMENTS.md`), archiveHeader + reqContent);
   }