feat: entry update links existing secrets (link_secret_names)

- secrets-core: update flow validates and applies secret links
- secrets-mcp: MCP tool params and UI for managing links on edit
- Align errors and templates; minor crypto/.gitignore tweaks

Made-with: Cursor

crates/secrets-core/src/error.rs

@@ -21,6 +21,9 @@ pub enum AppError {
     #[error("Concurrent modification detected")]
     ConcurrentModification,
 
+    #[error("Decryption failed — the encryption key may be incorrect")]
+    DecryptionFailed,
+
     #[error(transparent)]
     Internal(#[from] anyhow::Error),
 }