DX: add .editorconfig and optional pre-commit hooks mirroring the ruff config #12

What's missing

CONTRIBUTING.md documents a four-command quality gate (ruff check --fix, ruff format, ty check, plus the lychee link check), but contributors must remember to run it by hand, and non-Python files (YAML, Markdown) have no whitespace/end-of-line convention even though CI checks Markdown.

The repo has no .editorconfig and no .pre-commit-config.yaml. All the style decisions already exist in pyproject.toml (line-length = 100, target-version = "py311", double quotes, space indentation) — this issue is only about surfacing them to editors and git hooks.

Proposed change

1. .editorconfig mirroring the ruff settings: UTF-8, LF line endings, final newline, trim trailing whitespace, 4-space indent for Python, max_line_length = 100; 2-space indent for YAML.
2. .pre-commit-config.yaml running ruff check --fix and ruff format via ruff-pre-commit, pinned to the ruff version in uv.lock.
3. A short "optional pre-commit hooks" subsection in CONTRIBUTING.md (installation is uv run pre-commit install or pipx run pre-commit install — keep it optional; CI remains the gate).

Definition of done

• Both config files present; hook versions match the locked ruff version.
• CONTRIBUTING.md documents the optional setup.
• Running the hooks on the current tree produces no diff (i.e. the config matches what CI already enforces).

Validation

uv run ruff check --fix && uv run ruff format   # should be a no-op after the PR
lychee --no-progress --include-fragments \
  --exclude '^https://github\.com/Hebbian-Robotics/hflow/(issues|security/advisories/new)$' \
  --exclude-path references/mcap-spec.md \
  --exclude-path references/foxglove-CompressedVideo.proto .