chore: drop non-code junk (pi-mono-reference, runtime state, root duplicates)

Remove files that should never have been tracked:

- pi-mono-reference/: 688-file copy of an upstream monorepo, zero imports,
  used only as offline human reference
- build/electron-builder.runtime.json: environment-specific build artifact
  with hardcoded local paths
- state/cancel-signal-state.json: runtime state written by the app
- prompt.txt, logo.svg (root): exact duplicates of the canonical copies
  under .planning/archive/ and assets/

Add state/ to .gitignore so future runtime state stays out.

pi-mono-reference/packages/coding-agent/examples/extensions/with-deps/package-lock.json

@@ -1,31 +0,0 @@
-{
-  "name": "pi-extension-with-deps",
-  "version": "1.31.5",
-  "lockfileVersion": 3,
-  "requires": true,
-  "packages": {
-    "": {
-      "name": "pi-extension-with-deps",
-      "version": "1.31.5",
-      "dependencies": {
-        "ms": "^2.1.3"
-      },
-      "devDependencies": {
-        "@types/ms": "^2.1.0"
-      }
-    },
-    "node_modules/@types/ms": {
-      "version": "2.1.0",
-      "resolved": "https://registry.npmjs.org/@types/ms/-/ms-2.1.0.tgz",
-      "integrity": "sha512-GsCCIZDE/p3i96vtEqx+7dBUGXrc7zeSK3wwPHIaRThS+9OhWIXRqzs4d6k1SVU8g91DrNRWxWUGhp5KXQb2VA==",
-      "dev": true,
-      "license": "MIT"
-    },
-    "node_modules/ms": {
-      "version": "2.1.3",
-      "resolved": "https://registry.npmjs.org/ms/-/ms-2.1.3.tgz",
-      "integrity": "sha512-6FlzubTLZG3J2a/NVCAleEhjzq5oxgHyaCU9yYXvcLsvoVaHJq/s5xXI6/XXP6tz7R9xAOtHnSO/tXtF3WRTlA==",
-      "license": "MIT"
-    }
-  }
-}