feat: add markdown lint infrastructure

[offich]

Description

Adds Markdown linting infrastructure using markdownlint-cli2 via pnpm.
A GitHub Actions workflow is introduced to automatically check Markdown files
under CHANGELOG.md, docs/, and .claude/ on every push.

What was done

• Added .markdownlint.jsonc with project-specific rule configuration (adjusted MD007/MD013/MD014/MD024/MD041)
• Added package.json + pnpm-lock.yaml to manage markdownlint-cli2 as a devDependency
• Added mise.toml to pin the Node.js version (25.9.0)
• Added .github/workflows/docs-analysis.yml to run lint automatically on CI when target Markdown files are changed
• Added node_modules to .gitignore
• Added mise.toml, package.json, pnpm-lock.yaml, CLAUDE.md, node_modules/, and docs/ to .pubignore to exclude them from pub.dev releases
• Fixed existing list marker in CHANGELOG.md to comply with markdownlint rules

What was NOT done

• Creating the docs/ directory or writing Markdown documentation (to be handled separately)
• Setting up a pre-commit hook for local lint checks (CI only for now)

Operation Confirmation

Setup & Execution

1. Run mise install to set up Node.js
2. Run pnpm install to install dependencies
3. Run pnpm run lint:markdown to manually execute Markdown lint

Notes / Related URLs

N/A