feat: add OS compatibility checks before running platform-specific logic

- lib/common.sh: add require_local_os, require_remote_os, require_remote_pkg_manager
- setup/macbook.sh: require macOS (Darwin)
- setup/unraid.sh: require remote is Linux
- setup/fedora.sh: require remote is Linux + has dnf (RPM-based)
- manage_runner.sh: native runner add/remove requires macOS
- run_all.sh: control plane must be macOS
- preflight.sh: 3 new checks (1: local=macOS, 2: Unraid=Linux, 3: Fedora=Linux+dnf)
- phase5_migrate_pipelines.sh: fix sed -i to be portable (no macOS-only syntax)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

lib/common.sh

@@ -123,6 +123,55 @@ require_vars() {
   done
 }
 
+# ---------------------------------------------------------------------------
+# OS compatibility checks
+# ---------------------------------------------------------------------------
+
+# Verify the local machine is running the expected OS.
+# Usage: require_local_os "Darwin" "This script requires macOS"
+#   os_type: "Darwin" for macOS, "Linux" for Linux
+# Checks `uname -s` against the expected value.
+require_local_os() {
+  local expected="$1" msg="${2:-This script requires ${1}}"
+  local actual
+  actual="$(uname -s)"
+  if [[ "$actual" != "$expected" ]]; then
+    log_error "$msg"
+    log_error "Detected OS: $actual (expected: $expected)"
+    return 1
+  fi
+}
+
+# Verify a remote machine (via SSH) is running the expected OS.
+# Usage: require_remote_os "UNRAID" "Linux" "Unraid must be a Linux machine"
+#   host_key: SSH host key (UNRAID, FEDORA) — uses ssh_exec
+#   os_type: expected `uname -s` output
+require_remote_os() {
+  local host_key="$1" expected="$2" msg="${3:-Remote host $1 must be running $2}"
+  local actual
+  actual="$(ssh_exec "$host_key" "uname -s" 2>/dev/null)" || {
+    log_error "Cannot determine OS on ${host_key} (SSH failed)"
+    return 1
+  }
+  if [[ "$actual" != "$expected" ]]; then
+    log_error "$msg"
+    log_error "${host_key} detected OS: $actual (expected: $expected)"
+    return 1
+  fi
+}
+
+# Check if a remote host has a specific package manager available.
+# Usage: require_remote_pkg_manager "FEDORA" "dnf" "Fedora requires dnf"
+require_remote_pkg_manager() {
+  local host_key="$1" pkg_mgr="$2"
+  local msg="${3:-${host_key} requires ${pkg_mgr}}"
+  if ! ssh_exec "$host_key" "command -v $pkg_mgr" &>/dev/null; then
+    log_error "$msg"
+    log_error "${host_key} does not have '${pkg_mgr}' — is this the right machine?"
+    return 1
+  fi
+}
+
 # ---------------------------------------------------------------------------
 # SSH
 # ---------------------------------------------------------------------------