Add initial CLAUDE.md for AI-assisted development

[BryanFauble]

Summary

• Adds root CLAUDE.md (95 lines) and 4 module-level CLAUDE.md files (170 lines total) capturing non-obvious conventions and patterns that AI assistants cannot infer from the code alone
• Root: async-to-sync decorator system, protocol classes, dataclass models with fill_from_dict(), concrete Java type mapping, mixin composition, layered architecture, testing conventions
• synapseclient/models/: new model checklist, standard fields, _last_persistent_instance lifecycle, EnumCoercionMixin, fill_from_dict() pattern, @otel_trace_method usage
• synapseclient/api/: function signature conventions, REST call patterns, pagination helpers, new service file checklist
• synapseclient/core/: async_to_sync internals, retry strategies, credentials chain, upload/download resilience, concrete types registration
• tests/: async-only test convention, socket blocking, schedule_for_cleanup(), fixture scoping
• All commands verified from CI configs (build.yml, pytest.ini, .pre-commit-config.yaml)

Test plan

• Root file under 150 lines (95 lines)
• All module files under 150 lines (54, 45, 38, 33 lines)
• Every line passes the quality test: "If I remove this, will Claude mess up?"
• Commands are verbatim from config files
• No duplication with README.md content
• No linter/formatter rules restated (those are enforced by ruff/black/isort/bandit configs)
• Module files only contain patterns that differ from or extend the root CLAUDE.md

🤖 Generated with Claude Code