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>

README.md

@@ -1,3 +1,63 @@
 # config
 
-config is a tiny library to streamline configuration loading and validation using [viper](https://github.com/spf13/viper) and [validator](https://github.com/go-playground/validator).
+Typed configuration loading with [viper](https://github.com/spf13/viper) and [validator](https://github.com/go-playground/validator).
+
+## Install
+
+```bash
+go get gitea.auvem.com/go-toolkit/config
+```
+
+## Quick start
+
+```go
+type Schema struct {
+    AppName string `mapstructure:"appName" validate:"required"`
+}
+
+dir, err := config.RootDir(".env.yml", 1)
+if err != nil {
+    log.Fatal(err)
+}
+
+m := config.NewManager[Schema](".env", "yaml", dir).WithOpts(&config.ManagerOpts[Schema]{
+    EnvPrefix: "MYAPP",
+    PostLoad: func(cfg *Schema) error {
+        // derived fields — validated again after PostLoad
+        return nil
+    },
+})
+
+if err := m.Load(); err != nil {
+    log.Fatal(err)
+}
+
+cfg := m.C() // or m.Config()
+_ = cfg.AppName
+```
+
+## Load vs C
+
+- `Load()` returns errors explicitly — use in `main` during startup
+- `C()` and `Config()` lazy-load and panic on failure — convenient after startup
+
+## PostLoad hooks
+
+PostLoad runs after the first validation pass. The schema is **re-validated** after PostLoad so derived fields cannot bypass constraints.
+
+## RootDir
+
+`RootDir(name, depth)` returns `(string, error)`. `MustRootDir` panics on failure (for package-level `var` initialization).
+
+## Breaking changes (V1)
+
+| Before | After |
+|--------|-------|
+| `PostLoad func(*Manager[T])` | `PostLoad func(*T)` |
+| `PreLoad func(*Manager[T])` | `PreLoad func(*T)` |
+| `Manager.R` field | use `C()` / hook `*T` parameter |
+| `RootDir(...) string` (panic) | `RootDir(...) (string, error)`; use `MustRootDir` for panic |
+
+## Companion types
+
+Configuration structs in [dbx](https://gitea.auvem.com/go-toolkit/dbx) and [courier](https://gitea.auvem.com/go-toolkit/courier) include mapstructure/validate tags compatible with this package.