task: replace markdownlint with rumdl for markdown linting

[dgarros]

Need

Markdown linting currently relies on markdownlint-cli2, which is run through
npx and therefore drags a Node.js/npm toolchain into what is otherwise a
Python project. This adds setup friction locally and in CI (the dedicated
markdown-lint job), and it is slower than the newer Rust-based alternatives.

We want to move markdown linting to rumdl, a
fast Rust-based markdown linter, to remove the Node dependency for this check,
speed up linting, and align markdown tooling with the project's preference for
self-contained, Rust-backed tooling (as with ruff).

Context

markdownlint is wired into several places today, all of which are affected:

• .markdownlint-cli2.yaml and docs/.markdownlint.yaml — configuration
• tasks.py — lint_markdownlint task and the docs-generate --fix step
• .github/workflows/ci.yml — the markdown-lint job
  (DavidAnson/markdownlint-cli2-action)
• AGENTS.md / .agents/commands/pre-ci.md — contributor-facing references

The existing markdown style rules (see AGENTS.md "Markdown style") and the
current set of linted globs/exclusions should be preserved through the switch.

The rumdl configuration should live in pyproject.toml rather than a separate
config file, keeping tooling config consolidated with the rest of the project.

References

• rumdl: https://github.com/rvben/rumdl
• Current CI job: .github/workflows/ci.yml (markdown-lint)