v4.3.1

guard-core 4.0.x tracking: dependency floor to >=4.0.0, agent docs (v4.3.1)

• Compatibility - The guard-core floor moves from >=3.15.0 to >=4.0.0 in pyproject.toml (no upper bound), so this adapter tracks the guard-core 4.0.x engine line and is verified against guard-core 4.0.4. No adapter code changed: the delta since 4.3.0 is docs only, hence the patch bump. Since the adapter delegates all security behavior to the engine, no other change is required for the 4.0.x line; guard-core 4.0.0's own breaking changes (grammar-based secret redaction, exact-value require_headers() enforcement, excluded_detection_headers semantics) are documented in the guard-core changelog.
• Documentation - Added AGENTS.md/CLAUDE.md agent guidance and the djapi-guard package skill, and noted in the access-control decorator docs that an unknown bypass token now warns.
• Fixed (tests) - test_blocked_user_agent_pattern configured blocked_user_agents=[r".*crawler.*"], which guard-core 4.x's ReDoS validator rejects (adjacent broad unbounded quantifiers), so the suite cannot construct a SecurityConfig against the 4.0.x line. The test now blocks through [Cc]rawler/\d+\.\d+, which still exercises regex-pattern matching against a real user agent while satisfying the validator.