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>

go.mod

@@ -5,9 +5,11 @@ go 1.24.0
 require (
 	github.com/go-playground/validator/v10 v10.26.0
 	github.com/spf13/viper v1.20.1
+	github.com/stretchr/testify v1.10.0
 )
 
 require (
+	github.com/davecgh/go-spew v1.1.1 // indirect
 	github.com/fsnotify/fsnotify v1.9.0 // indirect
 	github.com/gabriel-vasile/mimetype v1.4.9 // indirect
 	github.com/go-playground/locales v0.14.1 // indirect
@@ -17,6 +19,7 @@ require (
 	github.com/leodido/go-urn v1.4.0 // indirect
 	github.com/niemeyer/pretty v0.0.0-20200227124842-a10e7caefd8e // indirect
 	github.com/pelletier/go-toml/v2 v2.2.4 // indirect
+	github.com/pmezard/go-difflib v1.0.0 // indirect
 	github.com/rogpeppe/go-internal v1.14.1 // indirect
 	github.com/sagikazarmark/locafero v0.9.0 // indirect
 	github.com/sourcegraph/conc v0.3.0 // indirect