feat(persist): default persistency path to systemd $STATE_DIRECTORY

[viktorbeck98]

Task

#190

Description

PersistConfig.path defaulted to the CWD-relative "./state". Under systemd
(where the service CWD is typically / unless WorkingDirectory= is set), this
resolves to /state — the wrong location, and writable only by root. Worse, the
resulting save failure is swallowed as a logger.warning in PersistencySaver.save(),
so the detector keeps running and silently persists nothing.

This PR makes the default environment-aware:

• PersistConfig.path now defaults to systemd's $STATE_DIRECTORY (set by
  StateDirectory= in the unit file, which also creates the directory with the
  correct service-user ownership), falling back to ./state outside systemd.
• A colon-separated $STATE_DIRECTORY (systemd's format for multiple
  StateDirectory= entries) uses the first path.
• An explicit PersistConfig(path=...) still wins — default_factory only runs
  when path is omitted, so no existing caller changes.

Implemented as a one-line Field(default_factory=...); no hardcoded system path,
which keeps the library portable across containers, non-root, and dev.

Docs (docs/detectors.md): added a "Running under systemd" note with a unit-file
example and updated the path field table.

Out of scope (worth separate issues): an XDG_STATE_HOME fallback for
non-systemd user daemons, and making PersistencySaver.save() fail fast / escalate
instead of silently swallowing permanent save failures.

How Has This Been Tested?

Added unit tests in detectmatelibrary_tests/test_common/test_persist_config.py
(class TestPersistConfigStateDirectoryDefault), written test-first:

• default reads $STATE_DIRECTORY when set,
• takes the first entry of a colon-separated list,
• falls back to ./state when unset,
• explicit path= overrides the env var.

Also pinned $STATE_DIRECTORY unset in the existing test_defaults (the change
made it env-sensitive). uv run pytest green: 19 passing in the persist-config
suite, 143 across the persistency-related suites. prek hooks (mypy, flake8,
bandit, …) all pass.

Checklist

• This Pull-Request goes to the development branch.
• I have successfully run prek locally.
• I have added tests to cover my changes.
• I have linked the issue-id to the task-description.
• I have performed a self-review of my own code.

[thorinaboenke]

what happens when STATE_DIRECTORY="", or if it starts with a colon? overwrites the path with an empty string, then the complete path will become /MyDetector and also have permission errors (root)

[thorinaboenke]

Suggested change

	default_factory=lambda: os.environ.get("STATE_DIRECTORY", "./state").split(":")[0]
	default_factory=lambda: (os.environ.get("STATE_DIRECTORY") or "./state").split(":")[0]