fix(kyberforge): keep .github/plugin/marketplace.json synced

apm.yml only declares claude and codex marketplace output profiles --
codex writes a differently-shaped file to .agents/plugins/marketplace.json,
so nothing regenerates the legacy Copilot CLI path at
.github/plugin/marketplace.json. It was hand-synced once during the #90
conversion and had drifted (missing the category field added when codex
output was enabled) with no gate to catch it.

scripts/sync-marketplace-mirror.sh keeps it byte-identical to the compiled
.claude-plugin/marketplace.json, wired as a pre-push check so it can't go
stale silently again.

Refs: #90

scripts/sync-marketplace-mirror.sh

@@ -0,0 +1,45 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+# .claude-plugin/marketplace.json is apm's compiled Claude marketplace output (see
+# apm.yml's marketplace.outputs.claude). GitHub Copilot CLI's manifest lookup accepts
+# that same file at .claude-plugin/marketplace.json directly, but also has a legacy
+# convention path at .github/plugin/marketplace.json (see
+# plugins/kyberforge/docs/research/docs/github-copilot-plugins/marketplace.md) -- and
+# CONTEXT.md documents that path as a mirror of the Claude output, not a separate apm
+# output profile (apm only ships "claude" and "codex" mappers; codex writes a
+# differently-shaped file to .agents/plugins/marketplace.json, not this path). This
+# script keeps that legacy mirror byte-identical to .claude-plugin/marketplace.json
+# instead of letting it silently drift (see issue #90 comment thread).
+
+REPO_ROOT="$(git rev-parse --show-toplevel 2>/dev/null || pwd)"
+SRC="$REPO_ROOT/.claude-plugin/marketplace.json"
+DST="$REPO_ROOT/.github/plugin/marketplace.json"
+
+usage() {
+  echo "Usage: $0 [--check]" >&2
+  exit 1
+}
+
+CHECK=0
+if [[ "${1:-}" == "--check" ]]; then
+  CHECK=1
+  shift
+fi
+[[ $# -eq 0 ]] || usage
+
+if [[ ! -f "$SRC" ]]; then
+  exit 0
+fi
+
+if [[ "$CHECK" -eq 1 ]]; then
+  if [[ ! -f "$DST" ]] || ! diff -q "$SRC" "$DST" >/dev/null 2>&1; then
+    echo "DRIFT $DST: out of sync with .claude-plugin/marketplace.json" >&2
+    echo "Fix: bash scripts/sync-marketplace-mirror.sh" >&2
+    exit 1
+  fi
+  exit 0
+fi
+
+mkdir -p "$(dirname "$DST")"
+cp "$SRC" "$DST"