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/orchestrator.rs

@@ -17,7 +17,7 @@ use crate::conversation::ConversationParty;
 use crate::ids::{AgentId, NodeId, SkillId};
 use crate::live_state::WorkStatus;
 use crate::mailbox::TicketId;
-use crate::skill::SkillScope;
+use crate::skill::{SkillKind, SkillScope};
 
 /// Current schema version for orchestration capabilities injected into an
 /// agent's effective runtime context.
@@ -39,6 +39,9 @@ pub struct AssignedSkillSnapshot {
     pub name: String,
     /// One-line affordance description shown in the model context.
     pub description: String,
+    /// Explicit nature of the assigned skill capability.
+    #[serde(default)]
+    pub kind: SkillKind,
     /// Position in the agent assignment list, preserving manifest order.
     pub assignment_index: u32,
 }