chore: add development conventions and model-routing config

Documents TDD/BDD/DDD methodology and Conventional Commits in CLAUDE.md,
and adds Claude Code model-switching setup (haiku/opus subagents, sonnet default).

CLAUDE.md

@@ -0,0 +1,13 @@
+# CLAUDE.md
+
+## Development methodology
+Apply these as much as the task reasonably allows; skip only for changes too trivial to warrant them (e.g. a one-line config tweak).
+
+- **TDD** — write a failing test before the implementation code for any new behavior; keep red-green-refactor tight.
+- **BDD** — state acceptance criteria as Given/When/Then before implementing a feature; use an executable BDD framework where the stack has one, otherwise plain test names/comments.
+- **DDD** — model code around the domain's real concepts and language (entities, value objects, aggregates, bounded contexts), not around technical/CRUD structure. Keep domain logic isolated from infrastructure/framework code.
+
+## Git
+- Use [Conventional Commits](https://www.conventionalcommits.org/) for every commit: `<type>(<scope>): <description>`.
+- Types: `feat`, `fix`, `docs`, `style`, `refactor`, `test`, `chore`, `build`, `ci`, `perf`.
+- One logical change per commit.