Add --no-minify flag to the opencode CLI build script

Title:
Add --no-minify flag to the opencode CLI build script

Description:
packages/opencode/script/build.ts already accepts --sourcemaps, --single,
--baseline, --skip-install, --skip-embed-web-ui, but there is no way to
opt out of minification. When a regression only reproduces against the
unminified bundle (readable stack traces, exact symbol names, dev-only
breakage hidden by the minifier), the only current option is to edit
build.ts and revert before committing. A --no-minify sibling flag keeps the
default behaviour and gives contributors a one-shot escape hatch.

Plugins: n/a
OpenCode version: dev (dc4449df0d)

Steps to reproduce:

1. cd packages/opencode && bun run script/build.ts --no-minify is not a
   recognised flag; build still runs with minify: true.
2. Edit build.ts and change minify: true to minify: false; the next
   build uses the unminified bundle.

Expected behaviour:

• bun run script/build.ts --no-minify should produce a non-minified bundle
  without source edits.
• Default behaviour (no flag) should be unchanged: minify: true as before.

Actual behaviour:

• --no-minify is silently ignored today.

Operating System: n/a
Terminal: n/a

Related / duplicate check: no related upstream issue found.