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/custom-provider-gitlab-duo/package.json

@@ -1,16 +0,0 @@
-{
-  "name": "pi-extension-custom-provider-gitlab-duo",
-  "private": true,
-  "version": "1.18.5",
-  "type": "module",
-  "scripts": {
-    "clean": "echo 'nothing to clean'",
-    "build": "echo 'nothing to build'",
-    "check": "echo 'nothing to check'"
-  },
-  "pi": {
-    "extensions": [
-      "./index.ts"
-    ]
-  }
-}