[Python] fd5 log CLI command

[gerchowl]

Parent: #161

Goal

Add fd5 log CLI subcommand to display the audit trail.

Spec

• fd5 log <file> — display audit history, newest first
• Output format (git-log style):

  commit sha256:def456...
  Parent: sha256:abc123...
  Author: Lars Gerchow <orcid:0000-0002-1825-0097>
  Date:   2026-03-02T14:30:00Z
  
      Updated calibration factor
  
      M /sensors/temperature.calibration_factor  1.0 → 1.05
  

• fd5 log <file> --json — machine-readable JSON output
• Exit 0 if log exists, exit 1 if no audit log

TDD

• Test output format matches spec
• Test empty log
• Test --json flag
• Test multiple entries display in reverse order