fix(test): stop the systemd tests from deleting the real ~/.config/systemd/user

[ytjohn]

Fixes #29.

test/install/systemd.test.ts creates a temp home under tmpdir() but never points HOME at it, then calls the real resolveTelePiInstallContext(). Since that resolver derives its paths from process.env.HOME || homedir() (src/paths.ts), every path the suite touches belongs to whoever runs the tests. Running npm test therefore:

1. deleted ~/.config/systemd/user recursively — rmSync(path.dirname(ctx.serviceUnitPath), { recursive: true, force: true }), which removes every unit file the user has there, not just telepi's, along with default.target.wants/ and timers.target.wants/;
2. overwrote the live telepi.service with ExecStart/StandardOutput paths inside the temp dir that afterEach then deletes, leaving the unit pointing at files that no longer exist;
3. ran real systemctl --user daemon-reload / enable / restart telepi.service from the reconcile test.

Changes

• Stub HOME to the per-test temp home in beforeEach, and restore process.env in afterEach — the same pattern test/install.test.ts already uses.
• Stop deleting a directory to prove auto-creation. The test now points serviceUnitPath at a not-yet-created path under tempDir and asserts it is under tempDir, so the destructive shape cannot return if the env stub is ever lost.
• Mock node:child_process for this file so reconcile/getStatus cannot drive the real service manager.

Verification

• npx vitest run test/install/systemd.test.ts — 13 passed.
• Full npm test — 32 files, 441 passed / 1 skipped.
• Ran the full suite with my real HOME, checksumming ~/.config/systemd/user before and after: unchanged, and telepi's ExecMainStartTimestamp did not move (no restart).

Happy to split the mock into its own commit, or to leave the rmSync test shape alone and rely on the HOME stub only, if you prefer a more minimal change.

[benedict2310]

Reviewed the complete diff and added deterministic success-path and exact failure/status assertions in 77b830f. Independent local Codex review found no defects. Local build and coverage pass: 443 tests, 86.87% lines/statements, 81.01% branches, 90.27% functions. All 14 focused tests also pass in a network-isolated, non-root Podman container. The temporary HOME, non-destructive directory creation test, and child-process mock should stay together. Awaiting CI for the updated head before merge.