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

knip.json

@@ -0,0 +1,17 @@
+{
+  "$schema": "https://unpkg.com/knip@5/schema.json",
+  "entry": [
+    "src/cli.ts",
+    "src/plugin-api.ts",
+    "src/plugin-api/unstable.ts",
+    "extensions/pi-web.ts",
+    "pi-web-plugins/**/pi-web-plugin.ts",
+    "scripts/*.mjs"
+  ],
+  "project": [
+    "src/**/*.ts",
+    "extensions/**/*.ts",
+    "pi-web-plugins/**/*.ts"
+  ],
+  "ignoreExportsUsedInFile": true
+}