feat: add Docker Compose support for Discord bot deployment

- Add Dockerfile with multi-stage build for picoclaw
- Add docker-compose.discord.yml for Discord bot service
- Add docker-compose.yml for agent mode service
- Add .env.example with environment variable template
- Add .dockerignore for optimized builds
- Update README.md with Docker Compose section and language switch
- Add README.ja.md (Japanese documentation)
- Update .gitignore with Docker-related entries

.gitignore

@@ -1,5 +1,7 @@
 # Binaries
+# Go build artifacts
 bin/
+build/
 *.exe
 *.dll
 *.so
@@ -10,12 +12,20 @@ bin/
 /picoclaw-test
 
 # Picoclaw specific
+
+# PicoClaw
 .picoclaw/
 config.json
 sessions/
 build/
 
 # Coverage
+
+# Secrets & Config (keep templates, ignore actual secrets)
+.env
+config/config.json
+
+# Test
 coverage.txt
 coverage.html