fix: resolve four frontend/backend issues

- Fix ProjectsPage tests by wrapping renders in MemoryRouter (9 passing)
- Improve session auto-naming to 'project / repo / tool' format
- Add missing /users/me/sessions endpoint for sidebar session loading
- Handle git history 500s: catch RuntimeError in endpoints, graceful empty repo handling
- Add git status badge and discard-changes button to FileEditor toolbar

Quality gates: tsc pass, build pass, Python syntax pass

apps/api/src/schemas/tool_instance.py

@@ -15,3 +15,27 @@ class CreateInstanceRequest(BaseModel):
     config_profile_id: str | None = Field(
         default=None, description="Optional config profile ID to apply to the instance"
     )
+
+
+class SessionItemResponse(BaseModel):
+    """Lightweight session summary for sidebar and dashboard."""
+
+    model_config = {"extra": "ignore"}
+
+    id: str = Field(description="Session (tool instance) ID")
+    display_name: str = Field(description="Display name of the session")
+    tool_type_name: str = Field(description="Name of the tool type")
+    tool_icon: str | None = Field(default=None, description="Icon URL for the tool type")
+    tool_type_interfaces: list[str] = Field(default_factory=list, description="Supported interfaces")
+    repository_name: str = Field(description="Name of the repository")
+    repository_id: str = Field(description="Repository ID")
+    project_name: str = Field(description="Name of the project")
+    project_id: str = Field(description="Project ID")
+    status: str = Field(description="Current status")
+    url: str | None = Field(default=None, description="Access URL")
+
+
+class SessionListResponse(BaseModel):
+    """Response wrapping a list of session summaries."""
+
+    sessions: list[SessionItemResponse]