feat(bd-3ke): add title truncation and key escaping for GitLab-to-Beads bridge

- Add truncate_title() function for bead titles (max 60 chars with ellipsis)
- Add escape_project() to replace / with :: in mapping keys for filesystem safety
- Add InvalidInput error code for validation errors
- Add comprehensive tests for truncation, escaping, and Unicode handling

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src-tauri/src/error.rs

@@ -46,6 +46,7 @@ pub enum McErrorCode {
     // General errors
     IoError,
     InternalError,
+    InvalidInput,
 }
 
 impl McError {
@@ -116,6 +117,15 @@ impl McError {
             "bv CLI not found -- is beads installed?",
         )
     }
+
+    /// Create an invalid input error
+    pub fn invalid_input(message: impl Into<String>) -> Self {
+        Self {
+            code: McErrorCode::InvalidInput,
+            message: message.into(),
+            recoverable: false,
+        }
+    }
 }
 
 impl std::fmt::Display for McError {