docs: add root documentation convention with a worked example

[alexgwolff]

Summary

This PR establishes the root documentation convention (spec/acceptance/intent per module) with a worked example, adds a cross-agent docs-review skill, and allows cross-agent agent config to be committed.

What does this pull request change?

• Defines the root documentation convention (docs/README.md): when a module needs spec.md, acceptance.md, and intent.md, plus a worked example under docs/example/
• Adds the docs-review skill under .agents/skills/ for cross-agent doc review
• Unblocks .agents/** in .gitignore to allow committing cross-agent config (now cross-agent config can be versioned)
• Updates BUILDING.md: the frontmatter exception now applies only to the root README.md (module READMEs now require frontmatter), and rewrites the agents rule to reflect the change above
• Skips CI on changes limited to **/*.md in the pull-request.yml workflow

How to test?

• Manually run the docs-review skill against docs/example/ and confirm it flags the worked example as compliant with the new convention

Other Notes and Links