feat: switch Qyburn to unified diff output + patch apply

Full-file replacement confirmed failing at all model sizes (tested up to
codestral:22b on 619-line file). Model hallucinates a new short file instead
of reproducing the original with changes applied.

New approach: model outputs only the unified diff (10-20 lines), applied
programmatically via `patch`. Works at any file size, uses ~154 tokens vs
~700+ previously, and succeeds on first attempt.

- Add `patch` binary to Dockerfile (apt install)
- Replace `_extract_code` with `_extract_diff`
- Add `_apply_diff` using subprocess patch
- Rewrite `_CODER_SYSTEM` to ask for diff format
- Rewrite `_build_prompt` to return plain string (no prefill dict)
- Rewrite `_ask_coder` to drop prefill assistant message
- Reduce num_predict 16384 → 4096 (diff output is short)
- Update `run_coding_loop` inner loop to apply diff then syntax-check result
- Update `_write_handoff` to show last diff instead of last full-file attempt

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

docker-compose.yml

@@ -0,0 +1,27 @@
+services:
+  qyburn:
+    build: .
+    container_name: qyburn-coder
+    restart: unless-stopped
+    ports:
+      - "8700:8700"
+    volumes:
+      - /opt/stacks:/opt/stacks:rw
+      - /opt/sites:/opt/sites:rw
+      - /opt/agent-os:/opt/agent-os:rw
+      - /var/run/docker.sock:/var/run/docker.sock
+      - /usr/bin/docker:/usr/bin/docker:ro
+      - /usr/libexec/docker/cli-plugins:/usr/libexec/docker/cli-plugins:ro
+    environment:
+      HODOR_URL: http://hodor-gateway:8200
+      OLLAMA_URL: http://172.27.40.20:11434
+      CODER_MODEL: phi4
+      STACKS_DIR: /opt/stacks
+      SITES_DIR: /opt/sites
+      AGENT_OS_DIR: /opt/agent-os
+    networks:
+      - proxy
+
+networks:
+  proxy:
+    external: true