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/sdk/11-sessions.ts

@@ -1,48 +0,0 @@
-/**
- * Session Management
- *
- * Control session persistence: in-memory, new file, continue, or open specific.
- */
-
-import { createAgentSession, SessionManager } from "@mariozechner/pi-coding-agent";
-
-// In-memory (no persistence)
-const { session: inMemory } = await createAgentSession({
-	sessionManager: SessionManager.inMemory(),
-});
-console.log("In-memory session:", inMemory.sessionFile ?? "(none)");
-
-// New persistent session
-const { session: newSession } = await createAgentSession({
-	sessionManager: SessionManager.create(process.cwd()),
-});
-console.log("New session file:", newSession.sessionFile);
-
-// Continue most recent session (or create new if none)
-const { session: continued, modelFallbackMessage } = await createAgentSession({
-	sessionManager: SessionManager.continueRecent(process.cwd()),
-});
-if (modelFallbackMessage) console.log("Note:", modelFallbackMessage);
-console.log("Continued session:", continued.sessionFile);
-
-// List and open specific session
-const sessions = await SessionManager.list(process.cwd());
-console.log(`\nFound ${sessions.length} sessions:`);
-for (const info of sessions.slice(0, 3)) {
-	console.log(`  ${info.id.slice(0, 8)}... - "${info.firstMessage.slice(0, 30)}..."`);
-}
-
-if (sessions.length > 0) {
-	const { session: opened } = await createAgentSession({
-		sessionManager: SessionManager.open(sessions[0].path),
-	});
-	console.log(`\nOpened: ${opened.sessionId}`);
-}
-
-// Custom session directory (no cwd encoding)
-// const customDir = "/path/to/my-sessions";
-// const { session } = await createAgentSession({
-//   sessionManager: SessionManager.create(process.cwd(), customDir),
-// });
-// SessionManager.list(process.cwd(), customDir);
-// SessionManager.continueRecent(process.cwd(), customDir);