feat: wizard entry points -- /setup command, claw setup subcommand, and RuntimeProviderConfig

The setup wizard was merged in PR #3017 but was orphaned -- it was not
declared as a module in main.rs, making it unreachable. Additionally,
the setup_wizard.rs imports RuntimeProviderConfig which did not exist
on upstream/main. This commit makes the wizard accessible and adds the
necessary RuntimeProviderConfig type.

Changes:
- Add RuntimeProviderConfig struct to runtime/src/config.rs with
  kind(), api_key(), base_url(), model() accessors.
- Add parse_optional_provider_config() to parse the provider object
  from merged settings JSON.
- Add provider() method to RuntimeConfig and RuntimeFeatureConfig.
- Export RuntimeProviderConfig, save_user_provider_settings,
  clear_user_provider_settings, and default_config_home from runtime
  crate public API (runtime/src/lib.rs).
- Add mod setup_wizard to rusty-claude-cli/src/main.rs.
- Add claw setup CLI subcommand.
- Add /setup slash command.
- Add Setup variant to SlashCommand enum.
- Add Setup to LocalHelpTopic enum.
- Add setup to diagnostic subcommand matching.
- Add subagentModel to TOP_LEVEL_FIELDS in config_validate.rs.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

rust/crates/runtime/src/config.rs

@@ -162,10 +162,46 @@ pub struct RuntimeFeatureConfig {
     trusted_roots: Vec<String>,
     api_timeout: ApiTimeoutConfig,
     rules_import: RulesImportConfig,
+    provider: RuntimeProviderConfig,
 }
 
 /// Controls which external AI coding framework rules are imported into the system prompt.
 #[derive(Debug, Clone, PartialEq, Eq, Default)]
+/// Stored provider configuration from the setup wizard.
+///
+/// Represents the `provider` section in `~/.claw/settings.json`, used as a
+/// fallback when environment variables are absent (3-tier resolution:
+/// env var > .env file > stored config).
+#[derive(Debug, Clone, PartialEq, Eq, Default)]
+pub struct RuntimeProviderConfig {
+    kind: Option<String>,
+    api_key: Option<String>,
+    base_url: Option<String>,
+    model: Option<String>,
+}
+
+impl RuntimeProviderConfig {
+    #[must_use]
+    pub fn kind(&self) -> Option<&str> {
+        self.kind.as_deref()
+    }
+
+    #[must_use]
+    pub fn api_key(&self) -> Option<&str> {
+        self.api_key.as_deref()
+    }
+
+    #[must_use]
+    pub fn base_url(&self) -> Option<&str> {
+        self.base_url.as_deref()
+    }
+
+    #[must_use]
+    pub fn model(&self) -> Option<&str> {
+        self.model.as_deref()
+    }
+}
+
 pub enum RulesImportConfig {
     /// Import from all supported frameworks when files are detected.
     #[default]
@@ -764,6 +800,7 @@ fn build_runtime_config(
         trusted_roots: parse_optional_trusted_roots(&merged_value)?,
         api_timeout: parse_optional_api_timeout_config(&merged_value)?,
         rules_import: parse_optional_rules_import(&merged_value)?,
+        provider: parse_optional_provider_config(&merged_value)?,
     };
 
     Ok(RuntimeConfig {
@@ -891,6 +928,13 @@ impl RuntimeConfig {
 }
 
 impl RuntimeFeatureConfig {
+    /// Parsed provider configuration (kind, apiKey, baseUrl, model) from
+    /// merged settings.
+    #[must_use]
+    pub fn provider(&self) -> &RuntimeProviderConfig {
+        &self.provider
+    }
+
     #[must_use]
     pub fn with_hooks(mut self, hooks: RuntimeHookConfig) -> Self {
         self.hooks = hooks;
@@ -2104,6 +2148,25 @@ fn parse_optional_rules_import(root: &JsonValue) -> Result<RulesImportConfig, Co
     }
 }
 
+fn parse_optional_provider_config(root: &JsonValue) -> Result<RuntimeProviderConfig, ConfigError> {
+    let Some(provider_value) = root.as_object().and_then(|object| object.get("provider")) else {
+        return Ok(RuntimeProviderConfig::default());
+    };
+    let Some(object) = provider_value.as_object() else {
+        return Ok(RuntimeProviderConfig::default());
+    };
+    let kind = optional_string(object, "kind", "provider")?.map(str::to_string);
+    let api_key = optional_string(object, "apiKey", "provider")?.map(str::to_string);
+    let base_url = optional_string(object, "baseUrl", "provider")?.map(str::to_string);
+    let model = optional_string(object, "model", "provider")?.map(str::to_string);
+    Ok(RuntimeProviderConfig {
+        kind,
+        api_key,
+        base_url,
+        model,
+    })
+}
+
 fn parse_filesystem_mode_label(value: &str) -> Result<FilesystemIsolationMode, ConfigError> {
     match value {
         "off" => Ok(FilesystemIsolationMode::Off),