feat(skill-awareness): T1→T5 — manifeste de skills + outil MCP idea_skill_read

Surface les skills assignés à un agent « à la MCP » : description sur l'entité
Skill (+ effective_description fallback), section « # Skills disponibles » haute
altitude dans le convention-file (mode MCP), outil read-only idea_skill_read
(résolution projet→global), use case ReadSkill (port SkillStore existant), et
câblage au composition root. Dump legacy du corps complet conservé en mode
sans-MCP (zéro régression). Rétro-compat index.json (serde default).

Tests : domain+application+infrastructure 1212 passed / 0 failed (23 ajoutés).
Reste : T6 (champ description front) + T7 (e2e après rebuild AppImage).

NB topologie : commit réalisé par l'orchestrateur car l'agent Git était
injoignable (bug de livraison cold-start) ; à faire relire/rebaser par Git.

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

crates/domain/src/orchestrator.rs

@@ -248,6 +248,17 @@ pub enum OrchestratorCommand {
         /// The reading party (handshake identity).
         requester: ConversationParty,
     },
+    /// Read a reusable skill's Markdown body **by name** (`idea_skill_read`,
+    /// feature « skills à la MCP »). Resolution is project-scope-first then global;
+    /// the body is returned inline. Read-only — no [`crate::fileguard::FileGuard`]
+    /// lease (skills are not mutated through this path).
+    ReadSkill {
+        /// Skill display name to resolve (case-insensitive).
+        name: String,
+        /// The party that issued the read (handshake identity). Carried for
+        /// symmetry/auditing with the other read tools; skill reads need no lease.
+        requester: ConversationParty,
+    },
     /// Write a memory note under the [`crate::fileguard::FileGuard`] (cadrage C7).
     /// Memory is project-shared; written directly under a write-lease.
     WriteMemory {
@@ -345,6 +356,10 @@ impl OrchestratorRequest {
                 slug: self.optional_slug(),
                 requester: self.requester_party(),
             }),
+            "skill.read" => Ok(OrchestratorCommand::ReadSkill {
+                name: self.require_name(action)?,
+                requester: self.requester_party(),
+            }),
             "memory.write" => Ok(OrchestratorCommand::WriteMemory {
                 slug: self.require("slug", action, self.slug.as_deref())?,
                 content: self.require("content", action, self.content.as_deref())?,