Add interactive setup wizard and persistent settings

- Add 7-step setup wizard: database, security, auth, model, embeddings, channels, heartbeat
- Store settings in ~/.ironclaw/settings.json with env var > settings > default priority
- Add OS keychain integration for secrets master key (macOS/Linux)
- Add `ironclaw config` CLI subcommand (list/get/set/reset/path)
- Expand Settings struct with all configuration fields
- Enhanced setup detection to auto-trigger wizard when needed

Co-Authored-By: Claude Opus 4.5 <[email protected]>

src/error.rs

@@ -47,6 +47,9 @@ pub enum ConfigError {
     #[error("Missing required environment variable: {0}")]
     MissingEnvVar(String),
 
+    #[error("Missing required configuration: {key}. {hint}")]
+    MissingRequired { key: String, hint: String },
+
     #[error("Invalid configuration value for {key}: {message}")]
     InvalidValue { key: String, message: String },