feat: allow relative target paths for git mounts

- Remove absolute path requirement from target_path validation
- Resolve relative paths against working_directory at instance startup
- Fall back to /home/user if no working_directory is configured
- Update frontend to allow relative target paths
- Update spec to document relative path support
- Update tests to allow relative paths and test path traversal rejection

apps/web/src/components/git-mount-editor.tsx

@@ -36,7 +36,6 @@ export const GitMountEditor = ({ mounts, onChange }: GitMountEditorProps) => {
   const validatePath = (path: string, isTarget: boolean): string | null => {
     if (!path) return isTarget ? "Target path is required" : null;
     if (path.includes("..")) return "Path cannot contain ..";
-    if (isTarget && !path.startsWith("/")) return "Target path must be absolute";
     if (!isTarget && path.startsWith("/")) return "Source path must be relative";
     return null;
   };
@@ -200,7 +199,7 @@ const GitMountForm = ({ mount, onSave, onCancel, validatePath, validateUrl, isNe
           placeholder="e.g., /app/config"
           className={errors.target_path ? "error" : ""}
         />
-        <span className="hint">Absolute path inside container</span>
+        <span className="hint">Absolute or relative path inside container (relative resolved against working dir)</span>
         {errors.target_path && <span className="error-text">{errors.target_path}</span>}
       </div>