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/04-skills.ts

@@ -1,52 +0,0 @@
-/**
- * Skills Configuration
- *
- * Skills provide specialized instructions loaded into the system prompt.
- * Discover, filter, merge, or replace them.
- */
-
-import {
-	createAgentSession,
-	createSyntheticSourceInfo,
-	DefaultResourceLoader,
-	SessionManager,
-	type Skill,
-} from "@mariozechner/pi-coding-agent";
-
-// Or define custom skills inline
-const customSkill: Skill = {
-	name: "my-skill",
-	description: "Custom project instructions",
-	filePath: "/virtual/SKILL.md",
-	baseDir: "/virtual",
-	sourceInfo: createSyntheticSourceInfo("/virtual/SKILL.md", { source: "sdk" }),
-	disableModelInvocation: false,
-};
-
-const loader = new DefaultResourceLoader({
-	skillsOverride: (current) => {
-		const filteredSkills = current.skills.filter((s) => s.name.includes("browser") || s.name.includes("search"));
-		return {
-			skills: [...filteredSkills, customSkill],
-			diagnostics: current.diagnostics,
-		};
-	},
-});
-await loader.reload();
-
-// Discover all skills from cwd/.pi/skills, ~/.pi/agent/skills, etc.
-const { skills: allSkills, diagnostics } = loader.getSkills();
-console.log(
-	"Discovered skills:",
-	allSkills.map((s) => s.name),
-);
-if (diagnostics.length > 0) {
-	console.log("Warnings:", diagnostics);
-}
-
-await createAgentSession({
-	resourceLoader: loader,
-	sessionManager: SessionManager.inMemory(),
-});
-
-console.log("Session created with filtered skills");