Add CLAUDE.md with coding and commit conventions

Author: marcarl

AGENTS.md

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+# Agent guidance
+
+See [CLAUDE.md](CLAUDE.md).

CLAUDE.md

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+# Claude Code guidance for sfs-processor
+
+## Commit messages
+
+Write commit messages in English, imperative mood, sentence case, no trailing period.
+
+Follow the pattern established in this repo:
+
+- Bug fixes: `Fix: <what was broken and how>`
+- New additions: `Add <thing being added>`
+- Updates/changes: `Update <thing being changed>`
+- Improvements: `Improve <what improved>`
+- Renames/moves: `Rename <old> to <new>`
+
+When a commit closes a GitHub issue, append `(#N)` at the end.
+
+Examples from this repo:
+
+```text
+Fix: YAML list corruption for forarbeten in frontmatter
+Add GitHub Pages publishing workflow
+Update README.md: Clarify target-date usage
+Improve HTML site upload scripts
+```
+
+## Code style
+
+- Python 3.10+ — use `match`, `X | Y` union types, and other modern syntax freely.
+- Line length: 120 characters (configured in `pyproject.toml`).
+- Lint and format with `ruff` before committing:
+
+```sh
+ruff check . --fix
+ruff format .
+```
+
+## Tests
+
+```sh
+python -m pytest test/ -v
+```
+
+All existing tests must pass. Tag new tests with the appropriate marker: `unit`, `integration`, `api`, or `slow`.
+
+## Architecture decisions
+
+Significant architectural choices are documented as ADRs in `docs/adr/`. If a change involves a non-obvious architectural trade-off, add or update an ADR.
+
+## Domain language
+
+`sfs-processor` converts Swedish legislation (SFS — Svensk författningssamling) from JSON to Markdown, HTML, and Git-history formats. Terms like *forarbeten*, *normtyp*, *giltighetstid*, and *SFS-nummer* are intentional Swedish legal vocabulary — do not translate or alter them.