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/tui/src/kill-ring.ts

@@ -1,46 +0,0 @@
-/**
- * Ring buffer for Emacs-style kill/yank operations.
- *
- * Tracks killed (deleted) text entries. Consecutive kills can accumulate
- * into a single entry. Supports yank (paste most recent) and yank-pop
- * (cycle through older entries).
- */
-export class KillRing {
-	private ring: string[] = [];
-
-	/**
-	 * Add text to the kill ring.
-	 *
-	 * @param text - The killed text to add
-	 * @param opts - Push options
-	 * @param opts.prepend - If accumulating, prepend (backward deletion) or append (forward deletion)
-	 * @param opts.accumulate - Merge with the most recent entry instead of creating a new one
-	 */
-	push(text: string, opts: { prepend: boolean; accumulate?: boolean }): void {
-		if (!text) return;
-
-		if (opts.accumulate && this.ring.length > 0) {
-			const last = this.ring.pop()!;
-			this.ring.push(opts.prepend ? text + last : last + text);
-		} else {
-			this.ring.push(text);
-		}
-	}
-
-	/** Get most recent entry without modifying the ring. */
-	peek(): string | undefined {
-		return this.ring.length > 0 ? this.ring[this.ring.length - 1] : undefined;
-	}
-
-	/** Move last entry to front (for yank-pop cycling). */
-	rotate(): void {
-		if (this.ring.length > 1) {
-			const last = this.ring.pop()!;
-			this.ring.unshift(last);
-		}
-	}
-
-	get length(): number {
-		return this.ring.length;
-	}
-}