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/ai/src/utils/sanitize-unicode.ts

@@ -1,25 +0,0 @@
-/**
- * Removes unpaired Unicode surrogate characters from a string.
- *
- * Unpaired surrogates (high surrogates 0xD800-0xDBFF without matching low surrogates 0xDC00-0xDFFF,
- * or vice versa) cause JSON serialization errors in many API providers.
- *
- * Valid emoji and other characters outside the Basic Multilingual Plane use properly paired
- * surrogates and will NOT be affected by this function.
- *
- * @param text - The text to sanitize
- * @returns The sanitized text with unpaired surrogates removed
- *
- * @example
- * // Valid emoji (properly paired surrogates) are preserved
- * sanitizeSurrogates("Hello 🙈 World") // => "Hello 🙈 World"
- *
- * // Unpaired high surrogate is removed
- * const unpaired = String.fromCharCode(0xD83D); // high surrogate without low
- * sanitizeSurrogates(`Text ${unpaired} here`) // => "Text  here"
- */
-export function sanitizeSurrogates(text: string): string {
-	// Replace unpaired high surrogates (0xD800-0xDBFF not followed by low surrogate)
-	// Replace unpaired low surrogates (0xDC00-0xDFFF not preceded by high surrogate)
-	return text.replace(/[\uD800-\uDBFF](?![\uDC00-\uDFFF])|(?<![\uD800-\uDBFF])[\uDC00-\uDFFF]/g, "");
-}