#7850: Add rumdl markdown linter to pre-commit hooks

## Summary - Add rumdl markdown linter to pre-commit hooks ## Notes - Rules with existing violations disabled initially to avoid blocking commits - Enable rules one by one and run `rumdl check --fix` to auto-fix issues - See rule documentation: https://rumdl.dev/RULES/ <!-- greptile_comment --> <h2>Greptile Overview</h2> <h3>Greptile Summary</h3> This PR adds a new `rumdl` Markdown linting hook to the repo’s pre-commit configuration and introduces a `.rumdl.toml` file to configure which Markdown rules are initially disabled. The change fits into the existing pre-commit workflow by adding another repo-backed hook alongside other linters (shellcheck, actionlint, etc.), with an initial configuration that avoids blocking commits due to existing Markdown rule violations. <h3>Confidence Score: 4/5</h3> - This PR is likely safe to merge; changes are isolated to pre-commit configuration. - Edits only add a new pre-commit hook and a linter config file, with no impact on runtime code. Main risk is misconfiguration (hook not running on intended files or running too broadly), which is easy to verify and adjust. - .pre-commit-config.yaml (verify hook file scoping/behavior), .rumdl.toml (ensure intended ruleset) <!-- greptile_other_comments_section --> _{(3/5) Reply to the agent's comments like "Can you suggest a fix for this @greptileai?" or ask follow-up questions!} **Context used:** - Context from `dashboard` - CLAUDE.md ([source](https://app.greptile.com/review/custom-context?memory=fd949e91-5c3a-4ab5-90a1-cbe184fd6ce8)) - Context from `dashboard` - AGENTS.md ([source](https://app.greptile.com/review/custom-context?memory=0d0c8278-ef8e-4d6c-ab21-f5527e322f13)) <!-- /greptile_comment -->