Add PR template and CLAUDE.md guide

.github/pull_request_template.md

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+<!--
+  Please write this PR in ENGLISH: summary, changes, testing notes, and related issue references.
+
+  Fill in the applicable sections below.
+  Do not invent information.
+  Only check testing items that were actually run.
+  Delete a section only if it truly does not apply.
+-->
+
+## Summary
+
+<!-- One or two sentences: what does this PR change, and why? -->
+
+## Type of change
+
+* [ ] Bug fix
+* [ ] New feature
+* [ ] Breaking change
+* [ ] Refactoring
+* [ ] Analyzer / diagnostic change
+* [ ] Tests
+* [ ] Documentation
+* [ ] Build / CI / tooling
+
+## Changes
+
+<!-- Bullet list of the concrete changes made in this PR. Keep it factual. -->
+
+*
+
+## Testing
+
+<!-- Check only the commands/tests that were actually run. Add details if something was not run. -->
+
+* [ ] `dotnet build FirstClassErrors.sln`
+* [ ] `dotnet test FirstClassErrors.sln`
+* [ ] Analyzer tests pass (`FirstClassErrors.Analyzers.UnitTests`)
+
+## Documentation
+
+<!-- State whether documentation was updated, or why no documentation change was needed. -->
+
+* [ ] Public API / error documentation updated
+* [ ] README / `doc/` updated
+* [ ] French translation (`doc/README.fr.md`) updated if user-facing behavior changed
+* [ ] No documentation change required
+
+## Related issues
+
+<!-- e.g. Closes #123 -->

CLAUDE.md

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+# FirstClassErrors — guide for Claude Code
+
+FirstClassErrors is a .NET library that treats errors as first-class,
+documented, and diagnosable concepts. Keep changes aligned with that goal:
+errors should stay structured, documented, and close to the code.
+
+## Language
+
+* The repository language is **English** by default:
+  source code, code comments, commit messages, branch names,
+  PR titles and descriptions, and issues.
+* The English documentation is canonical.
+* The French documentation in `doc/README.fr.md` is an intentional translation
+  and must stay in sync with the English documentation when user-facing behavior changes.
+* You may reply to me in French in the chat, but never write repository content in French
+  unless you are updating the French documentation.
+
+## Build & test
+
+* Target framework: **.NET Standard 2.0**.
+* Build: `dotnet build FirstClassErrors.sln`
+* Test: `dotnet test FirstClassErrors.sln`
+* Run the analyzer tests when touching analyzers:
+  `dotnet test FirstClassErrors.Analyzers.UnitTests`
+* Only report tests as passing if you actually ran the corresponding command.
+* If you did not run a relevant command, say so explicitly.
+
+## Project layout
+
+* `FirstClassErrors`               — main library
+* `FirstClassErrors.Analyzers`     — Roslyn analyzers (+ `.UnitTests`)
+* `FirstClassErrors.GenDoc`        — error-documentation generator (+ `.UnitTests`)
+* `FirstClassErrors.GenDoc.Worker` — background worker for doc generation
+* `FirstClassErrors.Cli`           — command-line tool
+* `FirstClassErrors.Usage`         — usage examples
+* `doc/`                           — documentation, including the French translation
+
+## Change guidelines
+
+* Keep changes small, focused, and aligned with the requested task.
+* Do not introduce new dependencies without a clear reason.
+* Do not make public API changes unless they are required by the task.
+* Treat renamed error codes, diagnostic IDs, and public types as breaking changes unless explicitly stated otherwise.
+* Preserve compatibility with **.NET Standard 2.0**.
+* Code style and inspection severities are defined in `FirstClassErrors.sln.DotSettings`
+  (ReSharper/Rider). Follow it; do not reformat code against these settings.
+
+## Error and documentation conventions
+
+* When you add or change an error, update its documentation accordingly.
+* When you change user-facing behavior, keep the English README and the French translation
+  (`doc/README.fr.md`) in sync.
+* When you change analyzers, update or add analyzer tests.
+* When you change diagnostics, keep diagnostic IDs, messages, documentation, and tests consistent.
+
+## Git and pull requests
+
+* Follow `.github/pull_request_template.md` for every pull request.
+* Do not open a pull request unless I explicitly ask for one.
+* PR titles, descriptions, commits, and branch names must be written in English.
+* In PR descriptions, do not invent testing results. Only check items that were actually run.