Add make lint-markdown target for local markdown linting

[isaacschepp]

Problem

PR #737 added markdownlint-cli2 CI. Local runs require typing npx --yes markdownlint-cli2 (documented in CONTRIBUTING.md). The Makefile has entries for every other lint/check:

make lint           # golangci-lint + website lint
make check-schemas  # Validate JSON schema files
make check-links    # Validate internal markdown links

Markdown lint is conspicuously missing.

Proposed Solution

Add two targets mirroring the existing pattern:

.PHONY: lint-markdown
lint-markdown: ## Lint markdown with markdownlint-cli2
	npx --yes markdownlint-cli2

.PHONY: lint-markdown-fix
lint-markdown-fix: ## Auto-fix markdown violations
	npx --yes markdownlint-cli2 --fix

Also wire lint-markdown into the umbrella make lint target so make lint runs both Go lint and markdown lint.

Update CLAUDE.md and CONTRIBUTING.md to mention the new Make target.

References

• PR ci: add markdownlint-cli2 for specification structural validation #737 (ci: add markdownlint-cli2 for specification structural validation) — landed the lint check without a Make target
• Makefile — current targets to mirror