Add binary build system for cross-platform distribution

- Add scripts/build.ts: Bun-based build script for generating binaries
- Support macOS (x64, ARM64, Universal), Windows (x64, ARM64), Linux (x64, ARM64)
- Add build:* npm scripts for each platform (build:mac, build:win, build:linux, etc)
- Configure electron-builder with platform-specific settings:
  * macOS: DMG + ZIP, Universal binaries, proper icon paths
  * Windows: NSIS installer + ZIP, configurable install directory
  * Linux: AppImage + DEB + tar.gz packages
- Add BUILD.md: Comprehensive build documentation with examples
- Update README.md: Add build instructions and reference BUILD.md
- Artifacts named: OpenCode Client-{version}-{os}-{arch}.{ext}
- Output directory: release/

Usage:
  bun run build:mac       # macOS Universal (Intel + Apple Silicon)
  bun run build:win       # Windows x64
  bun run build:linux     # Linux x64
  bun run build:all       # All platforms

scripts/build.ts

@@ -0,0 +1,84 @@
+#!/usr/bin/env bun
+
+import { $ } from "bun"
+import { existsSync } from "fs"
+import { join } from "path"
+
+const platforms = {
+  mac: {
+    args: ["--mac", "--x64", "--arm64", "--universal"],
+    description: "macOS (Intel, Apple Silicon, Universal)",
+  },
+  "mac-x64": {
+    args: ["--mac", "--x64"],
+    description: "macOS (Intel only)",
+  },
+  "mac-arm64": {
+    args: ["--mac", "--arm64"],
+    description: "macOS (Apple Silicon only)",
+  },
+  win: {
+    args: ["--win", "--x64"],
+    description: "Windows (x64)",
+  },
+  "win-arm64": {
+    args: ["--win", "--arm64"],
+    description: "Windows (ARM64)",
+  },
+  linux: {
+    args: ["--linux", "--x64"],
+    description: "Linux (x64)",
+  },
+  "linux-arm64": {
+    args: ["--linux", "--arm64"],
+    description: "Linux (ARM64)",
+  },
+  all: {
+    args: ["--mac", "--win", "--linux", "--x64", "--arm64"],
+    description: "All platforms (macOS, Windows, Linux)",
+  },
+}
+
+async function build(platform: string) {
+  const config = platforms[platform as keyof typeof platforms]
+
+  if (!config) {
+    console.error(`❌ Unknown platform: ${platform}`)
+    console.error(`\nAvailable platforms:`)
+    for (const [name, cfg] of Object.entries(platforms)) {
+      console.error(`  - ${name.padEnd(12)} : ${cfg.description}`)
+    }
+    process.exit(1)
+  }
+
+  console.log(`\n🔨 Building for: ${config.description}\n`)
+
+  try {
+    console.log("📦 Step 1/2: Building Electron app...\n")
+    await $`bun run build`
+
+    console.log("\n📦 Step 2/2: Packaging binaries...\n")
+    const distExists = existsSync(join(process.cwd(), "dist"))
+    if (!distExists) {
+      throw new Error("dist/ directory not found. Build failed.")
+    }
+
+    await $`bunx electron-builder ${config.args}`
+
+    console.log("\n✅ Build complete!")
+    console.log(`📁 Binaries available in: release/\n`)
+  } catch (error) {
+    console.error("\n❌ Build failed:", error)
+    process.exit(1)
+  }
+}
+
+const platform = process.argv[2] || "mac"
+
+console.log(`
+╔════════════════════════════════════════╗
+║   OpenCode Client - Binary Builder    ║
+╚════════════════════════════════════════╝
+`)
+
+await build(platform)