Add RCA for missing observer process

[wayyoungboy]

Summary

• add observer_process_missing RCA scene to detect missing/running observer processes on configured observer nodes
• collect lightweight evidence: observer process snapshot, observer version output, latest observer log tail, crash/signal keyword matches, and core file candidate list
• add unit tests for missing-process evidence, running-process start time, node filtering, and remote cleanup guard

Verification

• PYTHONPATH=. python -m pytest test/rca/test_observer_process_missing.py test/common/test_scene.py -q
• black --check -S -l 256 plugins/rca/observer_process_missing.py test/rca/test_observer_process_missing.py
• git diff --check

Fixes #146