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>

pyproject.toml

@@ -55,7 +55,7 @@ hbd = "hbd.server.cli:main"
 hbc = "hbd.client.main:main"
 
 [tool.setuptools]
-script-files = ["scripts/hb_install.sh"]
+script-files = ["scripts/hb_install.sh", "scripts/hbc_mini.py"]
 
 [tool.setuptools.packages.find]
 where = ["."]