chore: fix ruff lint warnings and signal handling edge cases

- Fix pre-existing ruff lint warnings (F401, F541, F841, E741)
- Exclude .md/.json/.yml from ruff format check
- Unblock SIGTERM/SIGINT via pthread_sigmask in early signal handler
- Do not restore SIG_DFL on unregister to prevent pending signal kills

pyproject.toml

@@ -49,4 +49,5 @@ target-version = "py312"
 quote-style = "double"
 indent-style = "space"
 skip-magic-trailing-comma = false
-line-ending = "auto"
+line-ending = "auto"
+exclude = ["*.md", "*.json", "*.yml", "*.yaml"]