docs: Added CLAUDE.md with AGENTS.md and references

[rmad17]

Changes

Adds agent-facing contributor guidance so AI coding assistants work within this SDK's security and compatibility rules. Docs and tooling config only, no library code changes.

Added

• CLAUDE.md: role, working principles, project structure, Always / Ask First / Never boundaries, security considerations (algorithm pinning, no token-derived fetches, MCD resolver trust, DPoP checks, token exchange guards), and short anchors into the reference files
• AGENTS.md: pointer to CLAUDE.md for agents that read AGENTS.md
• references/commands.md: install, test, coverage and lint commands that match CI
• references/testing.md: test layout, fixtures, async marker requirement, pytest-httpx mocking
• references/code-style.md: naming, error class pattern, # noqa: S105 usage for constants
• references/flow-map.md: entry points, helpers, tests and docs for each flow (Bearer/DPoP verification, connection tokens, custom token exchange, OBO, delegation, MCD, caching, anonymous callers)
• references/git-workflow.md: branch naming, commit prefixes, PR template sections
• references/pitfalls.md: Python 3.9 floor, ruff format not a CI gate, two dependency lists, custom_fetch scope, stale venvs
• references/docs-update.md: tracked docs inventory and code-to-docs update map

No public API, ApiClientOptions, error code, or dependency changes.