docs: add CONTRIBUTING.md

CONTRIBUTING.md

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+# Contributing to FHV-Latex
+
+Thank you for your interest in contributing to this project!
+
+---
+
+## Getting Started
+
+1. Fork the repository
+2. Clone your fork:
+   ```
+   git clone --recurse-submodules https://github.com/<your-username>/FHV-Latex.git
+   ```
+3. Create a feature branch from `main`:
+   ```
+   git checkout main
+   git pull
+   git checkout -b feature/your-feature
+   ```
+
+---
+
+## Commit Message Convention
+
+We follow [Conventional Commits v1.0.0](https://www.conventionalcommits.org).
+For general commit message best practices, see [cbea.ms/git-commit](https://cbea.ms/git-commit/) and [this summary](https://gist.github.com/robertpainsi/b632364184e70900af4ab688decf6f53).
+
+### Seven Golden Rules for Commits
+1. Separate subject from body with a blank line
+2. Limit the subject line to 72 characters
+3. Capitalize the subject line; do not end with a period
+4. Use the imperative mood: *"Add feature"* not *"Added feature"*
+5. Wrap body lines at 72 characters
+6. Explain *what* and *why* you changed something, not *how* (the diff shows that)
+7. When needed, use bullet points with hanging indents for lists
+
+### Format
+
+```
+<type>[optional scope]: <description>
+
+[Body — explain what and why, not how]
+
+[Footer — optional metadata]
+Resolves: #42
+See also: #10
+```
+
+Use lowercase after the type prefix. Limit the subject line to 72 characters. Use imperative mood (*"add"* not *"added"*).
+
+### Types
+
+| Type | Usage |
+|------|-------|
+| `feat` | New feature or functionality |
+| `fix` | Bug fix |
+| `docs` | Documentation only |
+| `style` | Formatting (no code behavior change) |
+| `refactor` | Code restructuring (no behavior change) |
+| `test` | Tests added or modified |
+| `ci` | CI/CD configuration |
+| `chore` | Maintenance tasks, dependency updates |
+
+### Example
+
+```
+fix(beamer): handle hyperref option clash with babel in TeX Live 2026
+
+hyperref={breaklinks=true} in \def\documentclassOptions caused babel
+to crash during macro expansion in babel v26.5. Moved the option to
+\hypersetup to avoid the conflict.
+```
+
+---
+
+## Branch Naming
+
+| Prefix | Usage |
+|--------|-------|
+| `feature/` | New feature |
+| `fix/` | Bug fix |
+| `docs/` | Documentation |
+| `refactor/` | Code restructuring |
+
+### Branch Rules
+
+- `main` is protected: no direct push, no force push, changes only via PR
+- Feature branches: force push allowed only when PR is in draft
+
+---
+
+## Clean History
+
+Use **rebase** (not merge) to keep your branch up to date:
+
+```bash
+git fetch origin
+git rebase origin/main
+```
+
+Before marking your PR as ready, clean up your commits:
+
+```bash
+git rebase -i origin/main    # squash fixup/review commits
+git push --force-with-lease --force-if-includes
+```
+
+**Merge strategy:** atomic features → squash merge; complex features with multiple meaningful commits → rebase cleanup + merge commit. Whoever merges is responsible for clean history.
+
+---
+
+## Building
+
+All 8 build variants must pass before a PR can be merged:
+
+```
+make clean && make FHVMODE=<mode> LANGUAGE=<language> [FHVTITLEPAGE=<titlepage>]
+```
+
+| Mode | Language | Titlepage | Description |
+|------|----------|-----------|-------------|
+| 1 | ngerman | | Documentation |
+| 2 | ngerman | | Thesis (default) |
+| 2 | english | | Thesis (english) |
+| 2 | ngerman | fhv | Thesis (FHV titlepage) |
+| 2 | english | fhv | Thesis (FHV titlepage, english) |
+| 3 | ngerman | | Summary |
+| 5 | ngerman | | Presentation (beamer) |
+| 9 | ngerman | | Paper |
+
+### Testing with Docker
+
+To test against the same TeX Live version used in CI:
+
+```bash
+docker run --rm -v "$(pwd):/work" -w /work texlive/texlive:latest bash -c \
+  'make clean && make FHVMODE=5 LANGUAGE=ngerman'
+```
+
+---
+
+## Beamer Mode (FHVMODE=5)
+
+The beamer document class has specific constraints — **test it explicitly**:
+
+- **Do not define commands that conflict with beamer builtins** (e.g. `\note`, `\pause`, `\alert`). Use unique names instead
+- **Do not pass `hyperref={...}` in `\documentclassOptions`** — beamer loads hyperref internally. Use `\hypersetup{...}` instead
+- **`scrhack` is not loaded for beamer** since `tocbasic` is unavailable
+- **`\usepackage{hyperref}` is skipped for beamer** — use `\hypersetup` to configure it
+
+---
+
+## Pull Requests
+
+- One logical change per PR — one commit per atomic change
+- **All 8 build variants must pass**
+- Do not rename existing public commands without a compelling reason
+
+### PR Description
+
+The PR body should answer:
+
+- **What** — what does this PR change?
+- **Why** — what is the motivation or problem being solved?
+- **How** — how was it tested? Which build variants were verified?
+
+### Review Checklist
+
+- [ ] All 8 build variants pass
+- [ ] Commit messages follow Conventional Commits
+- [ ] One topic per commit (atomic)
+- [ ] No unnecessary changes (whitespace, reformatting)
+
+---
+
+## Code Style
+
+- Use `%` comments to explain non-obvious LaTeX logic
+- Keep package loading order intentional — `fontenc` and `inputenc` must be loaded before `babel`
+- Use `\ifthenelse` for conditionals (or TeX primitives `\if...\fi` where `ifthen` is not yet loaded)
+
+---
+
+## Do's and Don'ts
+
+**Do:**
+- Small, atomic commits
+- Meaningful descriptions
+- Always keep all build variants green
+- Test FHVMODE=5 (beamer) explicitly
+
+**Don't:**
+- Force push on `main`
+- Mix multiple topics in one commit
+- Empty or generic messages ("fix stuff")
+- Bundle unrelated changes in one PR