docs(spec): promote eliminate-frontend-makefile backlog item to a brief

/kiro-discovery investigation of removing src/frontend/Makefile in favor
of Meson as the sole build system. Records the quick investigation's
findings (5 boundary seams, current-state evidence that build-cli/
build-server are already dead from /xce-build's perspective and
build-server is already broken today) and the user's confirmed decision
to fully remove the Makefile rather than keep a compat wrapper.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01AgYRU3zDTqNGV5Jk8aiKLg

.kiro/specs/eliminate-frontend-makefile/spec.json

@@ -0,0 +1,24 @@
+{
+  "feature_name": "eliminate-frontend-makefile",
+  "created_at": "2026-08-27T00:00:00Z",
+  "updated_at": "2026-08-27T00:00:00Z",
+  "language": "en",
+  "phase": "brief",
+  "description": "Remove src/frontend/Makefile entirely, consolidating Go builds (apiserver, xcecli) and their tests onto Meson's tools/go_build wrapper. Closes a real, already-realized duplicate version-fingerprint-computation risk (roadmap.md backlog item, recorded 2026-08-21). Path C single-scope investigation via /kiro-discovery, 2026-08-27.",
+  "approvals": {
+    "requirements": {
+      "generated": false,
+      "approved": false
+    },
+    "design": {
+      "generated": false,
+      "approved": false
+    },
+    "tasks": {
+      "generated": false,
+      "approved": false
+    }
+  },
+  "ready_for_implementation": false,
+  "blocked_on": []
+}