fix: create stub bridge/ dir in dependency caching layer

Hatchling's force-include requires bridge/ to exist at build time.
The dependency caching step now stubs both nanobot/ and bridge/.

https://claude.ai/code/session_011C1h1NERqqZp4ht3Pqpwkc

Dockerfile

@@ -16,9 +16,9 @@ WORKDIR /app
 
 # Install Python dependencies first (cached layer)
 COPY pyproject.toml README.md LICENSE ./
-RUN mkdir -p nanobot && touch nanobot/__init__.py && \
+RUN mkdir -p nanobot bridge && touch nanobot/__init__.py && \
     uv pip install --system --no-cache . && \
-    rm -rf nanobot
+    rm -rf nanobot bridge
 
 # Copy the full source and install
 COPY nanobot/ nanobot/