refactor(reconcile): simplify cache and sync flows

[veerendra2]

This PR refactors composeflux's reconciliation code to improve simplicity, memory management, and runtime safety:

1. Eliminate Stateful Struct Cache: Removes the cache and cacheMu fields from Reconciler and deletes cache.go entirely. Env/secrets are loaded statelessly on demand via loadEnvAndConfig and passed down to stack discovery.
2. Defensive Label Verification: Adds explicit safety check inside isManagedStack to prevent any possible nil map access.
3. Optimize & Clean Sync: Uses in-memory checks against discovered configurations to validate startup order directory presence.
4. Deterministic Deployment Ordering: Extracts ordering logic to the pure, testable helper orderStacks that applies the configured StartupOrder followed by a deterministic alphabetical fallback.