fix(ci): exclude external URLs from internal link check

fix: add blank lines around fenced code block in team-template.md

ci: add markdownlint config, fix CLAUDE.md formatting

ci: add CI workflow — markdown lint and internal link check

fix: remove @ sigil from matrix_uri examples, fix blob URL

fix: review fixes — frontmatter contradiction, A2A fields, clarity

Initial specs repo: Card specification, skill format, team template, ATP overview