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>

manage_runner.sh

@@ -189,6 +189,9 @@ add_docker_runner() {
 # and unreliable for long-running background services.
 # ---------------------------------------------------------------------------
 add_native_runner() {
+  # Native runners use launchctl + macOS-specific paths — must be macOS
+  require_local_os "Darwin" "Native runner deployment requires macOS (uses launchctl)"
+
   log_info "Deploying native runner '${RUNNER_NAME}' on local machine..."
 
   # Resolve ~ to actual home directory for local execution
@@ -278,6 +281,9 @@ remove_docker_runner() {
 # remove_native_runner — Unload launchd service + remove binary + plist
 # ---------------------------------------------------------------------------
 remove_native_runner() {
+  # Native runners use launchctl + macOS-specific paths — must be macOS
+  require_local_os "Darwin" "Native runner removal requires macOS (uses launchctl)"
+
   log_info "Removing native runner '${RUNNER_NAME}' from local machine..."
 
   # Resolve ~ to actual home directory