Clean up remaining dead code and dependencies

- Replace unused 'codemirror' barrel dep with explicit @codemirror/{state,
  view,commands,language} sub-packages that were imported but unlisted
- Remove unused exports: writeNamespacedQuery, targetWorkspacePathForRun,
  isWorkspaceDeletionRun, piWebConfigDir, duplicate parsePiWebRuntimeResponse
- Remove unused types: WorkspacePanelFiles, WorkspacePanelHost,
  GetActiveSession, SaveAttachmentsResponse, QueryValues
- Drop now-unused re-exports/imports cascading from the above
- Add knip config + 'npm run knip' script for ongoing dead-code detection

src/server/sessions/sessionRuntimeStore.ts

@@ -2,5 +2,3 @@ export interface ActiveSession<TRuntime> {
   runtime: TRuntime;
   unsubscribe: () => void;
 }
-
-export type GetActiveSession<TRuntime> = (sessionId: string) => Promise<ActiveSession<TRuntime>>;