chore(lint): add markdownlint-cli2 configuration

[mgratzer]

Parent: #1

Summary

Add markdownlint-cli2 as a linter for all markdown files, runnable via npx with no committed dependencies.

Problem / Motivation

No automated way to catch formatting inconsistencies, broken heading hierarchy, trailing whitespace, or style drift across 13 markdown files (~2,200 lines).

Proposed Solution

Add markdownlint-cli2 with a project-specific configuration file, run via npx.

Implementation Details

1. Create .markdownlint-cli2.jsonc with rules tuned for this project:
   • Disable MD013 (line length) — SKILL.md files have long template lines
   • Disable MD033 (inline HTML) — README.md uses centered HTML headers
   • Disable MD041 (first-line heading) — SKILL.md files start with YAML frontmatter
   • Enable all other default rules
2. Add lint command to CLAUDE.md commands section: npx markdownlint-cli2 "**/*.md"
3. Update docs/development.md to document the lint command
4. Fix any existing lint violations found on first run

Alternatives Considered

• package.json with lint script — Adds dependency management to a prompt-only repo. Not worth it for a single tool.
• mdformat (Python) — Auto-formatter that may aggressively rewrite SKILL.md content. Less control over rules.

Acceptance Criteria

• .markdownlint-cli2.jsonc created with project-appropriate rules
• npx markdownlint-cli2 "**/*.md" runs cleanly with no violations
• CLAUDE.md commands section updated
• docs/development.md updated
• Existing markdown files pass linting