feat: v0.7.0 — AgentDef provider/model + install --provider/--model/--no-config : résolution flag > agent > défaut, erreur si provider inconnu (liste du registre), warning si modèle absent, résolution affichée en résumé et au --dry-run (closes #90)

- Providers réels livrés dans le catalogue par défaut (anthropic★/openai/deepseek/google) + hints provider sur claude-code/deepseek-harness
- Suppression par null dans l'overlay : am providers remove écrit <nom>: null (merge traite null comme suppression, set_path remplace le null avant de descendre)
- Cascade @secret (#89) : le provider déclaré de l'agent prime sur default_provider

src/config.rs

@@ -148,7 +148,7 @@ pub struct Settings {
     /// they live in the OS keyring under `providers/<name>/api_key`
     /// (issue #89).
     #[serde(default)]
-    pub providers: Option<BTreeMap<String, ProviderDef>>,
+    pub providers: Option<BTreeMap<String, Option<ProviderDef>>>,
 }
 
 /// One entry of the LLM provider registry (issue #88).
@@ -275,7 +275,10 @@ impl Settings {
 
     /// Look up one provider of the registry (issue #88).
     pub fn provider(&self, name: &str) -> Option<&ProviderDef> {
-        self.providers.as_ref().and_then(|m| m.get(name))
+        self.providers
+            .as_ref()
+            .and_then(|m| m.get(name))
+            .and_then(|p| p.as_ref())
     }
 
     /// Name of the default provider (issue #88).
@@ -331,6 +334,14 @@ pub struct AgentDef {
     /// CLI flag carrying the model name at launch (issue #71, e.g. "--model").
     #[serde(default)]
     pub model_arg: Option<String>,
+    /// Preferred provider of this agent (issue #90, e.g. "anthropic" for
+    /// claude-code). Falls back to settings.default_provider when unset.
+    #[serde(default)]
+    pub provider: Option<String>,
+    /// Preferred model for this agent (issue #90). Falls back to the
+    /// provider's default_model when unset.
+    #[serde(default)]
+    pub model: Option<String>,
     #[serde(default)]
     pub tags: Vec<String>,
     /// When false, the agent is listed but cannot be installed locally.
@@ -588,6 +599,10 @@ pub struct InstallSpec {
     /// Commands executed after a successful install (PATH includes the bin dir).
     #[serde(default)]
     pub post_install: Vec<String>,
+    /// The agent accepts a post-install provider configuration (issue #91).
+    /// False for agents without any API (issue #90).
+    #[serde(default = "default_true")]
+    pub configurable: bool,
 }
 
 impl InstallSpec {
@@ -815,12 +830,21 @@ pub fn merge(base: &mut Config, overlay: Config) {
         s.default_provider = o.default_provider;
     }
     // Providers merge key by key (issue #88): an overlay adds or replaces
-    // one provider without wiping the others.
+    // one provider without wiping the others. An explicit `null` in the
+    // overlay DELETES the provider (the standard YAML overlay pattern) so a
+    // provider shipped in the default catalog can be removed by the user.
     if let Some(overlay_providers) = o.providers {
         match s.providers.as_mut() {
             Some(map) => {
                 for (k, v) in overlay_providers {
-                    map.insert(k, v);
+                    match v {
+                        Some(def) => {
+                            map.insert(k, Some(def));
+                        }
+                        None => {
+                            map.remove(&k);
+                        }
+                    }
                 }
             }
             None => s.providers = Some(overlay_providers),
@@ -900,6 +924,10 @@ pub fn validate(config: &Config) -> Vec<String> {
         }
         if let Some(providers) = &config.settings.providers {
             for (name, p) in providers {
+                // A null entry is a deletion marker (issue #90): skip it.
+                let Some(p) = p else {
+                    continue;
+                };
                 if p.base_url.trim().is_empty() {
                     problems.push(format!("provider '{name}': 'base_url' is required"));
                 }