docs: slim CLAUDE.md to what the code and docs don't say

[allen0099]

Rewrites CLAUDE.md from about 12 KB (126 lines) to about 4.6 KB. It keeps only what an agent cannot learn from one file, one grep or --help: invariants that span modules, test pitfalls and contribution conventions. It points to docs/ for the rest.

Kept (condensed)

• Commands. Plain uv run pytest does not enforce the 90% floor. Redis and Memcached tests are opt-in and flush their server. tox -e lowest. The docs build needs the docs group. CI also runs mypy tests/mypy scripts, which pre-commit does not.
• Invariants:
  • cache key format and separator escaping;
  • @cache fails open;
  • which callers fall back to MemoryBackend and which call BackendProxy.get();
  • Memcached key enumeration is a no-op, while clear() runs flush_all;
  • every backend must override the atomic primitives;
  • TTL/delta validation runs before I/O;
  • key prefixes.
• Tests:
  • filterwarnings = error;
  • close_network_clients;
  • examples/ are tested and included in the docs;
  • checking that a test can fail.
• Conventions:
  • changelog fragments;
  • zh-TW docs mirroring;
  • no hand-edited version;
  • deprecate first, remove in the next minor.

Removed, and where to find it instead

Removed	Source
Per-subsystem descriptions (cache flow, CacheManager, sessions, state, lock)	Module docstrings; docs/
Per-backend implementation details (Lua scripts, CAS retries, pooling)	fastapi_cachex/backends/*.py docstrings; docs/BACKENDS.md "Atomic backend primitives"
Generic commands (run one test, ruff, mypy, tox per version)	docs/DEVELOPMENT.md; tox.ini; --help
ruff / mypy / pydocstyle config details	pyproject.toml; enforced by pre-commit
pytest options beyond filterwarnings	pyproject.toml [tool.pytest.ini_options]
Mentions of SessionMiddleware, the UserSessionDep alias and the memcache extra	Stale: removed in 0.4.0 (#398–#403)