feat(skills): capacités agent découvrables via idea_list_agents

Ajoute SkillKind (Workflow/Reference) sur Skill, extrait le use case
ResolveAgentCapabilities à partir des SkillRef assignés, et enrichit
idea_list_agents d'un champ additif capabilities (name, description,
kind) — remplace l'exposition de SkillRef opaques par un inventaire de
capacités interrogeable, source commune avec le bloc « Skills
disponibles » injecté au lancement (#119).

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

crates/domain/src/skill.rs

@@ -27,6 +27,17 @@ pub enum SkillScope {
     Project,
 }
 
+/// Nature of a skill when surfaced as an agent capability.
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash, Serialize, Deserialize, Default)]
+#[serde(rename_all = "camelCase")]
+pub enum SkillKind {
+    /// Procedural workflow the agent may execute on demand.
+    #[default]
+    Workflow,
+    /// Reference material the agent may consult selectively.
+    Reference,
+}
+
 /// A reusable workflow assignable to one or more agents.
 ///
 /// Invariants enforced here:
@@ -45,6 +56,10 @@ pub struct Skill {
     /// [`Skill::effective_description`] falls back to the first line of the body.
     #[serde(default)]
     pub description: Option<String>,
+    /// Explicit skill nature. Defaults to [`SkillKind::Workflow`] for legacy
+    /// skill JSON/index rows that predate this field.
+    #[serde(default)]
+    pub kind: SkillKind,
     /// Markdown body — the workflow injected into an agent's convention file.
     pub content_md: MarkdownDoc,
     /// Scope (selects the backing store).
@@ -74,6 +89,7 @@ impl Skill {
             id,
             name,
             description: None,
+            kind: SkillKind::Workflow,
             content_md,
             scope,
         })
@@ -90,6 +106,13 @@ impl Skill {
         self
     }
 
+    /// Returns this skill with an explicit [`SkillKind`] set (builder).
+    #[must_use]
+    pub const fn with_kind(mut self, kind: SkillKind) -> Self {
+        self.kind = kind;
+        self
+    }
+
     /// The description to surface for this skill, with a deterministic fallback.
     ///
     /// Returns [`Skill::description`] when present and non-blank (trimmed);
@@ -122,7 +145,8 @@ impl Skill {
     pub fn with_content(&self, content_md: MarkdownDoc) -> Result<Self, DomainError> {
         Ok(
             Skill::new(self.id, self.name.clone(), content_md, self.scope)?
-                .with_description(self.description.clone()),
+                .with_description(self.description.clone())
+                .with_kind(self.kind),
         )
     }
 }
@@ -230,22 +254,26 @@ mod tests {
 
     #[test]
     fn deserialize_legacy_skill_without_description_defaults_to_none() {
-        // A skill JSON written before `description` existed must deserialise with
-        // `description: None` thanks to `#[serde(default)]`.
+        // A skill JSON written before `description`/`kind` existed must
+        // deserialise with additive defaults.
         let id = uuid::Uuid::from_u128(7);
         let json =
             format!(r##"{{"id":"{id}","name":"legacy","contentMd":"# body","scope":"global"}}"##);
         let parsed: Skill = serde_json::from_str(&json).unwrap();
         assert_eq!(parsed.description, None);
+        assert_eq!(parsed.kind, SkillKind::Workflow);
         assert_eq!(parsed.name, "legacy");
     }
 
     #[test]
-    fn serde_round_trip_with_description() {
-        let original = skill("# body").with_description(Some("affordance".to_owned()));
+    fn serde_round_trip_with_description_and_kind() {
+        let original = skill("# body")
+            .with_description(Some("affordance".to_owned()))
+            .with_kind(SkillKind::Reference);
         let json = serde_json::to_string(&original).unwrap();
         let back: Skill = serde_json::from_str(&json).unwrap();
         assert_eq!(back, original);
         assert_eq!(back.description.as_deref(), Some("affordance"));
+        assert_eq!(back.kind, SkillKind::Reference);
     }
 }