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.

Dockerfile

@@ -284,8 +284,10 @@ RUN if [ "${INSTALL_OMOS}" = "true" ]; then \
       unzip -o /tmp/bun.zip -d /tmp/bun && \
       mv /tmp/bun/bun-linux-${BUN_ARCH}/bun /usr/local/bin/bun && \
       chmod +x /usr/local/bin/bun && \
+      ln -sf bun /usr/local/bin/bunx && \
       rm -rf /tmp/bun /tmp/bun.zip && \
       bun --version && \
+      test -L /usr/local/bin/bunx && \
       npm install -g oh-my-opencode-slim@${OMOS_VERSION}; \
     fi