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.

entrypoint-user.sh

@@ -53,7 +53,7 @@ OMOS_CONFIG="$CONFIG_DIR/oh-my-opencode-slim.json"
 OMOS_CONFIG="$CONFIG_DIR/oh-my-opencode-slim.json"
 
 if [ "${ENABLE_OMOS:-false}" = "true" ]; then
-  if ! command -v bunx &>/dev/null; then
+  if ! command -v bun &>/dev/null; then
     echo "WARNING: ENABLE_OMOS=true but bun is not installed."
     echo "Rebuild with: docker compose build --build-arg INSTALL_OMOS=true"
   elif [ ! -f "$OMOS_CONFIG" ]; then
@@ -70,7 +70,7 @@ if [ "${ENABLE_OMOS:-false}" = "true" ]; then
       OMOS_SKILLS_FLAG="no"
     fi
 
-    bunx oh-my-opencode-slim@latest install \
+    bun x oh-my-opencode-slim@latest install \
       --no-tui \
       --tmux="${OMOS_TMUX_FLAG}" \
       --skills="${OMOS_SKILLS_FLAG}"
@@ -87,7 +87,7 @@ if [ "${ENABLE_OMOS:-false}" = "true" ]; then
       OMOS_SKILLS_FLAG="yes"
       [ "${OMOS_SKILLS:-true}" = "false" ] && OMOS_SKILLS_FLAG="no"
 
-      bunx oh-my-opencode-slim@latest install \
+      bun x oh-my-opencode-slim@latest install \
         --no-tui \
         --tmux="${OMOS_TMUX_FLAG}" \
         --skills="${OMOS_SKILLS_FLAG}" \