dev: add markdownlint and docs-analysis workflow

[offich]

Description

Introduced markdownlint to maintain Markdown file quality, and added a docs-analysis CI workflow to run automated checks.

What was done

• Added markdownlint-cli2 as a devDependency (Node.js managed via pnpm + mise)
• Added rule configuration in .markdownlint.jsonc (adjusted MD007/MD013/MD014/MD024/MD041)
• Added lint:markdown / lint:markdown:fix scripts to package.json
• Added GitHub Actions docs-analysis.yml workflow (runs lint on changes to CHANGELOG.md, .claude/**/*.md, etc.)
• Fixed list formatting in CHANGELOG.md to comply with markdownlint rules
• Added relevant entries to .gitignore and .pubignore

What was NOT done

• No impact on Dart/Flutter code
• Additional features such as auto-commenting on PRs are out of scope

Operation Confirmation

Prerequisites & Steps

# To run lint locally
mise install       # Install Node.js 25.9.0
pnpm install
pnpm run lint:markdown

CI runs automatically on changes to docs/, .claude/**/*.md, and CHANGELOG.md.

Notes / Related URLs

N/A