docs: add README badges and reusable-workflow call-graph diagram

[NWarila]

Summary

• Adds a two-badge row to README.md (Repo CI workflow status, License MIT), matching the packer-framework-template badge style ([![CI](…)](…) then [![License: MIT](…)](LICENSE)).
• Adds the first Mermaid diagram to this repo: docs/diagrams/reusable-workflow-call-graph.mmd — a flowchart showing a consumer repo calling all seven universal reusables by SHA, with the drift-gate relationship between reusable-repo-hygiene ↔ baseline-manifest.json and reusable-org-adr-auto-sync ↔ org-adr-manifest.json.
• Embeds the diagram inline in docs/explanation/repo-type-taxonomy-and-ci-focus.md with a source link per docs/reference/diagram-conventions.md.
• Allowlists docs/diagrams/reusable-workflow-call-graph.mmd in .gitignore (deny-all strategy requires explicit per-file entries).

Ruleset note (no change in this PR)

The "Default Branch Protection" ruleset on this repo is currently active but empty — no rules are configured inside it. It is vestigial and should be removed via terraform in a follow-on infrastructure change. This PR does not touch rulesets.

Validation

• python tools/check_docs_layout.py — passes
• python tools/check_adr_schema.py — passes
• python tools/check_baseline_manifest.py — passes
• npx markdownlint-cli2 "**/*.md" — 0 errors
• git status --short after staging — all 4 changed files tracked (none ignored)
• Diff scanned for AI bylines, "generated by", Co-authored-by — none present

Test plan

• Repo CI workflow passes (actionlint, markdownlint, docs-layout, adr-schema, baseline-manifest, attribution-residue, smoke tests)
• README badges render correctly on the repo landing page
• Mermaid diagram renders in docs/explanation/repo-type-taxonomy-and-ci-focus.md on GitHub