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.