feat: add zero-friction ProofLoop demo

[FHoffarth]

Summary

• Adds proofloop demo subcommand to run a zero-dependency, 30-second offline showcase proving the ProofLoop core thesis.
• Demonstrates the three canonical dispositions:
  • BLOCKED: Agent claims phantom commit -> Pre-Gate blocks immediately, skips Judge LLM.
  • REVIEW_REQUIRED: Judge LLM votes ACCEPT, but evaluative claims force human review.
  • PASS: Pure deterministic evidence verified cleanly.
• Adds smart default for git verification profiles (--commit defaults to HEAD when omitted in git repositories).
• Improves report readability by displaying human-readable claim statements alongside technical IDs and actionable Resolution guidance.
• Remains strictly offline with zero network calls, zero API keys, and zero live model dependencies.
• Core proof laws and deterministic gate policies are completely preserved and untouched.

Validation

• 210 tests passed (python -m pytest -v -m "not integration")
• git diff --check clean with zero whitespace warnings
• Working tree clean