Clarify the OKF bundle conformance boundary

[1wgrumph]

Summary

• define one bundle root for each OKF conformance evaluation
• clarify which Markdown files enter the conformance corpus
• keep non-Markdown resources, links, path-valued fields, and symbolic-link targets from expanding that corpus
• align the cross-linking, conformance, and versioning sections with the boundary definition

Why

A repository or archive can contain multiple knowledge bundles alongside generated files, code, and other resources. Defining the conformance corpus makes validation consistent without turning resource resolution, filesystem access, or execution into OKF conformance requirements.

Validation

• python3 -m pytest — 39 passed
• git diff --check