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/biome.json

@@ -1,41 +0,0 @@
-{
-	"$schema": "https://biomejs.dev/schemas/2.3.5/schema.json",
-	"linter": {
-		"enabled": true,
-		"rules": {
-			"recommended": true,
-			"style": {
-				"noNonNullAssertion": "off",
-				"useConst": "error",
-				"useNodejsImportProtocol": "off"
-			},
-			"suspicious": {
-				"noExplicitAny": "off",
-				"noControlCharactersInRegex": "off",
-				"noEmptyInterface": "off"
-			}
-		}
-	},
-	"formatter": {
-		"enabled": true,
-		"formatWithErrors": false,
-		"indentStyle": "tab",
-		"indentWidth": 3,
-		"lineWidth": 120
-	},
-	"files": {
-		"includes": [
-			"packages/*/src/**/*.ts",
-			"packages/*/test/**/*.ts",
-			"packages/coding-agent/examples/**/*.ts",
-			"packages/web-ui/src/**/*.ts",
-			"packages/web-ui/example/**/*.ts",
-			"!**/node_modules/**/*",
-			"!**/test-sessions.ts",
-			"!**/models.generated.ts",
-			"!packages/web-ui/src/app.css",
-			"!packages/mom/data/**/*",
-			"!!**/node_modules"
-		]
-	}
-}