fix: validate config after PostLoad hooks

Re-run validator after PostLoad, return errors from RootDir, and add Config
alias for C(). Hooks receive *T instead of *Manager.

Co-authored-by: Cursor <cursoragent@cursor.com>

doc.go

@@ -0,0 +1,20 @@
+// Package config loads and validates typed configuration files using viper and
+// go-playground/validator.
+//
+// # Workflow
+//
+// Define a struct with mapstructure and validate tags, create a [Manager] with
+// [NewManager], optionally configure hooks via [Manager.WithOpts], then call
+// [Manager.Load] during startup. Use [Manager.C] or [Manager.Config] for quick
+// access after load.
+//
+// # Environment overrides
+//
+// Set [ManagerOpts.EnvPrefix] or call [Manager.WithEnvOverride]. Nested keys use
+// dots in YAML and underscores in env vars (for example MYAPP_DATABASE_USER).
+//
+// # Root directory discovery
+//
+// [RootDir] and [MustRootDir] locate a marker file by walking up from the working
+// directory—useful when config lives near a repo root.
+package config