fix(lint): make the markdownlint pre-commit hook pass on a pristine tree

[sbaerlocher]

Summary

• The markdownlint step of the lefthook pre-commit failed on the unmodified default branch: CHANGELOG.md produced 64 errors. Every commit staging a changelog entry was blocked and could only land with --no-verify, which also disables gitleaks, yamllint, ansible-lint and actionlint.
• Three separate causes, none of which required relaxing a rule. MD007: .editorconfig had no indent_size under [*.md], so Markdown inherited the global 4. Prettier reads .editorconfig and reindented nested bullets to 4, which MD007.indent=2 then rejected — the two hooks could not both be satisfied. Setting indent_size = 2 makes them agree, matching how arillso/action.playbook already solves this; .markdownlint.json keeps the org-wide indent: 2 unchanged.
• MD024: two genuinely duplicated headings, not the format-inherent repetition of Keep a Changelog. A second ### Added inside [Unreleased] and a second ### Removed inside [1.0.0] were merged into the existing sections, so siblings_only stays on and real duplicates keep being reported.
• MD041: CLAUDE.md is a bare @AGENTS.md import and the pull request template is a form fragment starting at H2. Both are headingless by construction and now carry a file-local markdownlint-disable-file.
• Reformatting is indentation-only outside CHANGELOG.md; no wording changed.

Test plan

• npx --yes -- markdownlint-cli2 $(git ls-files '*.md') — 0 issues across all 11 files
• npx --yes -- prettier --check over the hook globs (*.json, *.yml, *.yaml, *.md) — clean
• Commit lands without --no-verify, with gitleaks, markdownlint and prettier all passing
• git diff --ignore-all-space on AGENTS.md and the READMEs is empty, confirming indentation-only changes
• CI green

[claude]

No issues found.

Verified the fix end-to-end:

• .editorconfig indent_size = 2 for [*.md] is the correct root cause — no prettier config exists, so editorconfig drives prettier's markdown indent, and it now agrees with .markdownlint.json MD007.indent = 2.
• Whitespace-ignored diff on AGENTS.md and all four READMEs is empty → indentation-only, no wording changed.
• CHANGELOG MD024 fixes are pure section merges: no content lost, and both moved blocks stay within the same version's Added/Removed semantics.
• MD041 disables are correctly scoped to the two headingless-by-construction files.
• The three untouched md files still pass: the only 4-space lines (plugins/README.md) are inside a fenced code block, which prettier/MD007 ignore.