feat: add hbc_mini single-file client; drop dead connections on protocol error

- scripts/hbc_mini.py: self-contained hbc with no external deps; uses
  /proc for CPU/memory/network on Linux, df for disk, JSON config
- hbc + hbc_mini: mark connection _dead and stop sending on protocol error
- README: document hbc_mini usage, config, and plugin availability
- pyproject.toml: include hbc_mini.py in script-files

Co-Authored-By: Claude Sonnet 4.6 (1M context) <noreply@anthropic.com>

README.md

@@ -441,6 +441,68 @@ plugins:
 
 All monitoring plugins default to 5-minute (300 second) intervals, but can be customized as needed.
 
+### hbc_mini — single-file client (no external dependencies)
+
+`scripts/hbc_mini.py` is a self-contained version of the heartbeat client that requires only Python 3.8+ and no external packages. Copy it to any host and run it directly — no virtualenv, no `pip install`.
+
+```bash
+# Basic usage
+python3 hbc_mini.py your-server.example.com
+
+# Run as daemon
+python3 hbc_mini.py -d your-server.example.com
+
+# Send a boot message
+python3 hbc_mini.py -b your-server.example.com
+
+# Send a one-off message
+python3 hbc_mini.py -m "maintenance starting" your-server.example.com
+```
+
+**Config:** `~/.hbc.json` (same keys as `~/.hbc.yaml`, JSON format). Example:
+
+```json
+{
+  "hb_port": 50003,
+  "interval": 30,
+  "plugins": {
+    "ping_monitor": {
+      "interval": 60,
+      "hosts": ["8.8.8.8", "192.168.1.1"]
+    },
+    "nagios_runner": {
+      "interval": 300,
+      "commands": [
+        {"name": "check_load", "command": "/usr/lib/nagios/plugins/check_load -w 5,4,3 -c 10,8,6"}
+      ]
+    }
+  }
+}
+```
+
+**Plugin availability:**
+
+| Plugin | Platform | Data source |
+|---|---|---|
+| `os_info` | all | `platform` stdlib |
+| `ping_monitor` | all | `ping` subprocess |
+| `nagios_runner` | all (not Windows) | subprocess |
+| `cpu_monitor` | Linux | `/proc/stat` |
+| `memory_monitor` | Linux | `/proc/meminfo` |
+| `disk_monitor` | Linux, macOS, BSD | `df -P` subprocess |
+| `network_monitor` | Linux | `/proc/net/dev` |
+
+**What is not available compared to the full `hbc`:**
+
+- No YAML config (use JSON instead)
+- No `filesystem_info` plugin
+- `cpu_monitor` does not report per-core usage or CPU frequency (no psutil)
+- Plugins cannot be loaded from external `.py` files — all plugins are compiled in
+
+Everything else — heartbeat protocol, ACK/CMD/UPD handling, `hb_install.sh`-based self-update, daemonize, syslog — is identical to the full client.
+
+---
+
 ## 🐞 Debugging in VS Code
 
 This repository includes a ready-to-use `.vscode/launch.json` with configurations to run or attach the VS Code debugger to `hbd`.