docs: clarify action receipt verification boundary

[carloshvp]

Summary

Refs #66.

• clarifies the action-receipt verification boundary for embodied workflows in the spec
• adds verification-guide taxonomy for accepted receipts, valid rejections, missing required receipts, and invalid receipts
• adds informative action-receipt fixture shapes without changing the TRACE schema

Validation

• .venv/bin/ruff check src tests
• .venv/bin/mypy src/agentrust_trace
• .venv/bin/pytest --cov=agentrust_trace --cov-report=term-missing
• git diff --check

Docs build note:

• .venv/bin/mkdocs build --strict --site-dir /tmp/trace-action-receipts-site-check currently fails before rendering changed pages because the existing MkDocs config uses docs_dir: ., which MkDocs rejects as the parent directory of the config file.