feat: make Epi Log ambient and traceable

[bearmug]

Summary

• Replace the post-hoc daily/report-first core with a small execution-time traceability loop.
• Keep intent → decision → change → evidence → outcome in existing work artifacts.
• Add materiality, permission, evidence, anti-gaming, and human-review guardrails.
• Move the detailed daily/trailing-seven-day workflow to optional references/audit-mode.md.
• Add dependency-free docs/link/front-matter validation and run it in GitHub Actions.

Validation

• python3 scripts/check-docs.py
• git diff --check
• Clean checkout validation: clone branch, rerun docs checks, verify clean status
• Evidence/read-only policy searches from the repository: passed
• Core reduced to 52 lines; it contains no manual daily/weekly run mode or default report storage.

Review notes addressed

The README paths and local links are checked by scripts/check-docs.py; the workflow runs the check on pull requests and pushes to main. The core defines no executable user commands, but the lightweight docs check protects the documented file references and Agent Skills front matter.