Validate deadman arming and make sysfs writes safe and reversible

[timkpaine]

Follow-ups from the review of #64 (merged while the review ran).

• arm confirms the timer with a query that must answer: a failed query after systemd-run is a DeadmanError, not "probably armed". is_armed keeps its conservative default for the pre-arm and disarm checks.
• --pattern "" (an empty shell variable) matched every lspci line; empty patterns are refused.
• Attribute writes open with O_NOFOLLOW and refuse a power directory that is a symlink, so a valid-looking device under a caller-supplied --sysfs-root cannot redirect a root write. The device entry itself is a symlink on a real sysfs by design and stays allowed.
• A write that fails part-way (permission, device gone) restores the attributes already changed and reports what it restored; the failure is a RuntimePmError with exit 1, not a traceback.
• Option ordering: the subparsers accept arm --timeout N, the form dau-build uses; --timeout N arm no longer parses. Stated here rather than supported.

[github-actions]

Test Results

43 tests  +4   43 ✅ +4   0s ⏱️ ±0s
 1 suites ±0    0 💤 ±0 
 1 files   ±0    0 ❌ ±0 

Results for commit 813e258. ± Comparison against base commit 91d67ef.

[codecov]

Codecov Report

❌ Patch coverage is 95.65217% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 94.32%. Comparing base (91d67ef) to head (813e258).

Files with missing lines	Patch %	Lines
dau_utils/pci_runtime_pm.py	89.18%	3 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #65      +/-   ##
==========================================
+ Coverage   94.14%   94.32%   +0.17%     
==========================================
  Files           6        6              
  Lines         495      581      +86     
  Branches       39       47       +8     
==========================================
+ Hits          466      548      +82     
- Misses         26       29       +3     
- Partials        3        4       +1     

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