test(cli): opt-in pre-built binary for ~3x faster subprocess spawns

\`bun run --conditions=browser src/index.ts\` pays ~15s of JIT + plugin
init + DB migration per subprocess spawn in isolation mode. A pre-built
binary cuts that to ~5s — most of which is now the SQLite \`:memory:\`
migration that runs regardless of execution mode.

Adds \`script/prebuild-test-cli.ts\` which wraps the existing build.ts
with \`--single --skip-embed-web-ui --skip-install\`, then symlinks the
platform-specific output to \`dist/test-cli/bin/opencode\` so the
harness has a stable path.

The harness (test/lib/cli-process.ts) reads OPENCODE_TEST_CLI_PATH and
spawns the binary directly when set; falls back to dev mode otherwise.
Strictly opt-in — default behavior, CI, and local iteration are
unchanged. Anyone who wants the speedup runs:

  bun script/prebuild-test-cli.ts
  export OPENCODE_TEST_CLI_PATH="\$PWD/dist/test-cli/bin/opencode"
  bun test test/cli/

Measured locally:
  Dev mode (default):  29.9s  (331 tests)
  Binary mode:         22.1s  (-26%, after one-time 2.8s build)

The win compounds as more subprocess tests are added — every new test
that hits DB migration saves ~10s vs dev mode.

packages/opencode/script/prebuild-test-cli.ts

@@ -0,0 +1,53 @@
+#!/usr/bin/env bun
+// Build a pre-compiled `opencode` binary for subprocess tests, then expose
+// it at `dist/test-cli/bin/opencode` for the harness to consume.
+//
+// Why: each `bun run --conditions=browser src/index.ts <cmd>` spawn pays
+// ~15s of JIT + plugin init + DB migration in isolation mode. The
+// pre-compiled binary cuts that to ~5s — a 3x improvement on subprocess
+// tests that touch the DB (mcp, providers list, etc.).
+//
+// Usage:
+//   bun script/prebuild-test-cli.ts
+//   export OPENCODE_TEST_CLI_PATH="$PWD/dist/test-cli/bin/opencode"
+//   bun test test/cli/
+//
+// The harness (see test/lib/cli-process.ts) reads OPENCODE_TEST_CLI_PATH; if
+// set, it spawns the binary directly instead of `bun run src/index.ts`. If
+// unset, it falls back to dev mode — so this script is strictly opt-in.
+//
+// Build cost amortizes after ~1 spawn that touches the DB. Recommended for
+// CI, manual `bun test test/cli/` runs, and any local iteration where the
+// CLI surface itself isn't under change. Skip for normal src/* editing — the
+// dev path picks up source changes without rebuild.
+import { $ } from "bun"
+import fs from "node:fs/promises"
+import path from "node:path"
+
+const dir = path.resolve(import.meta.dirname, "..")
+process.chdir(dir)
+
+const platform = process.platform === "win32" ? "win32" : process.platform === "darwin" ? "darwin" : "linux"
+const arch = process.arch === "x64" ? "x64" : process.arch === "arm64" ? "arm64" : "x64"
+const targetDir = path.join(dir, "dist", `opencode-${platform}-${arch}`)
+const binaryName = process.platform === "win32" ? "opencode.exe" : "opencode"
+const builtBinary = path.join(targetDir, "bin", binaryName)
+
+// Stable path the harness reads via OPENCODE_TEST_CLI_PATH. Symlinked so
+// the binary itself remains the platform-specific one (build.ts manages it).
+const stableBinary = path.join(dir, "dist", "test-cli", "bin", binaryName)
+
+console.log(`Building test CLI binary for ${platform}-${arch}...`)
+const start = Date.now()
+await $`bun script/build.ts --single --skip-embed-web-ui --skip-install`
+const buildMs = Date.now() - start
+console.log(`Build complete in ${buildMs}ms: ${builtBinary}`)
+
+await fs.mkdir(path.dirname(stableBinary), { recursive: true })
+await fs.rm(stableBinary, { force: true })
+await fs.symlink(builtBinary, stableBinary)
+console.log(`Symlinked stable path: ${stableBinary}`)
+console.log(``)
+console.log(`To use in tests:`)
+console.log(`  export OPENCODE_TEST_CLI_PATH="${stableBinary}"`)
+console.log(`  bun test test/cli/`)