Fix OMOS bunx detection

entrypoint-user.sh gated OMOS auto-install on 'command -v bunx', but
neither upstream bun installer nor our Dockerfile creates a bunx
symlink — only the bun binary exists on PATH. The check always failed
on a fresh OMOS image, printing 'ENABLE_OMOS=true but bun is not
installed.' even though bun was right there. Latent until now because
the only exercised path had a persisted oh-my-opencode-slim.json from
a prior install.

Fixes:
- Gate on 'command -v bun' instead of bunx.
- Call 'bun x oh-my-opencode-slim@latest install ...' (bun x is the
  real subcommand that actually works with only the bun binary).
- Add 'ln -sf bun /usr/local/bin/bunx' in the Dockerfile OMOS block
  so interactive users can still type bunx by habit.
- Smoke test asserts the bunx symlink exists on the OMOS variant.

Also verify 'test -L /usr/local/bin/bunx' as a build-time sanity check.
Can't use 'bunx --help' for that — bun exits 1 on help output even
though it prints the usage correctly.

scripts/smoke-test.sh

@@ -107,6 +107,7 @@ fi
 # bun: only in the omos variant
 if [ "$VARIANT" = "omos" ]; then
   run "bun (omos)"            "bun --version"
+  run "bunx symlink (omos)"   "test -L /usr/local/bin/bunx && readlink /usr/local/bin/bunx"
   # oh-my-opencode-slim is npm-installed globally (not a bun install);
   # verify it shows up in the global module list.
   run "oh-my-opencode-slim"   "npm ls -g --depth=0 2>/dev/null | grep oh-my-opencode-slim"