test(internal/config): cover defaults/env/YAML/precedence + harden SaveAtomic
Adds tests for internal/config covering defaults, env-var loading, YAML loading and precedence, plus reliability hardening of SaveAtomic (F_FULLFSYNC on darwin, explicit enc.Close, per-path mutex via sync.Map, unique tmp via os.CreateTemp, fsynced parent dir, new SaveAtomicCtx wrapper for cancellation). Fixes a precedence bug where envconfig.Process ran after yaml.Unmarshal so env silently overrode YAML; the order is now env-first / YAML-last per the documented CLI > YAML > env precedence. BREAKING: deployments that today set an env var to override a YAML key will see the YAML value win after this change. Closes #708 Signed-off-by: Ales Verbic <[email protected]>