feat(#384): Add Synapse + Element Web to docker-compose for dev

- Create docker-compose.matrix.yml as optional dev overlay
- Add Synapse homeserver config with shared PostgreSQL
- Add Element Web client config (port 8501)
- Add bot account setup script (docker/matrix/scripts/setup-bot.sh)
- Add Makefile targets: matrix-up, matrix-down, matrix-logs, matrix-setup-bot
- Document Matrix env vars in .env.example
- Synapse accessible at localhost:8008, Element at localhost:8501
- Usage: docker compose -f docker/docker-compose.yml -f docker/docker-compose.matrix.yml up

Refs #384

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.env.example

@@ -350,6 +350,28 @@ OLLAMA_MODEL=llama3.1:latest
 # Get your API key from: https://platform.openai.com/api-keys
 # OPENAI_API_KEY=sk-...
 
+# ======================
+# Matrix Dev Environment (docker-compose.matrix.yml overlay)
+# ======================
+# These variables configure the local Matrix dev environment.
+# Only used when running: docker compose -f docker/docker-compose.yml -f docker/docker-compose.matrix.yml up
+#
+# Synapse homeserver
+# SYNAPSE_CLIENT_PORT=8008
+# SYNAPSE_FEDERATION_PORT=8448
+# SYNAPSE_POSTGRES_DB=synapse
+# SYNAPSE_POSTGRES_USER=synapse
+# SYNAPSE_POSTGRES_PASSWORD=synapse_dev_password
+#
+# Element Web client
+# ELEMENT_PORT=8501
+#
+# Matrix bridge connection (set after running docker/matrix/scripts/setup-bot.sh)
+# MATRIX_HOMESERVER_URL=http://localhost:8008
+# MATRIX_ACCESS_TOKEN=<obtained from setup-bot.sh>
+# MATRIX_BOT_USER_ID=@mosaic-bot:localhost
+# MATRIX_SERVER_NAME=localhost
+
 # ======================
 # Logging & Debugging
 # ======================