feat: auto-install missing build dependencies (pnpm, bun, nfpm)

Build scripts now check for required tools before building and install
them automatically if missing. Handles both amd64 and arm64 host systems.

- pnpm: installed via corepack or npm
- bun: installed via official install script
- nfpm: downloaded from GitHub for the correct host architecture
- node_modules: runs pnpm install if missing

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

scripts/build-rpm.sh

@@ -19,6 +19,10 @@ source "$SCRIPT_DIR/arch-helper.sh"
 resolve_arch "${MCPCTL_TARGET_ARCH:-}"
 # Sets: NFPM_ARCH, BUN_TARGET, ARCH_SUFFIX
 
+# Check and install missing build dependencies
+source "$SCRIPT_DIR/ensure-deps.sh"
+ensure_build_deps
+
 echo "==> Running unit tests..."
 pnpm test:run
 echo ""