feat(backend): support des providers OpenCode cloud (#92)

Ajoute le catalogue statique de providers OpenCode (lot B3), le stockage
sécurisé des secrets (SecretStore + adapter infrastructure), et les
use cases SaveOpenCodeProviderProfile/DeleteProfile câblés en composition
root. Couvre le fix B1 et les tests de régression demandés par QA.

cargo build --workspace propre, cargo test --workspace -- --test-threads=1
intégralement vert.

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

crates/application/src/error.rs

@@ -7,7 +7,7 @@
 
 use domain::ports::{
     AgentSessionError, EmbedderError, FsError, GitError, MemoryError, ModelServerError,
-    ProcessError, PtyError, RemoteError, RuntimeError, StoreError,
+    ProcessError, PtyError, RemoteError, RuntimeError, SecretStoreError, StoreError,
 };
 use domain::{AgentId, NodeId};
 use domain::{IssueStoreError, SprintStoreError};
@@ -236,6 +236,14 @@ impl From<RemoteError> for AppError {
     }
 }
 
+impl From<SecretStoreError> for AppError {
+    /// Maps to [`AppError::Store`] — a [`domain::ports::SecretStore`] failure is a
+    /// persistence failure like any other store, coherent with [`StoreError`].
+    fn from(e: SecretStoreError) -> Self {
+        Self::Store(e.to_string())
+    }
+}
+
 impl From<AgentSessionError> for AppError {
     /// Maps a structured [`AgentSessionError`] (ARCHITECTURE §17.1) onto the single
     /// application error shape. `Start`/`Io`/`Decode`/`Timeout` are all execution