Fix mutable default arguments, shared RetryTimeout instance and wrong UPDATE error message

[claude-agentic-bot]

Summary

Small robustness cleanups found while reviewing the repo:

• git_host/base.py: get/list/create/update used mutable defaults (params={}, headers={}). Replaced with None and normalised inside the methods, so a mutation by a client can't leak between calls.
• git_host/base.py: update() raised 'CREATE is not supported…'; it now says UPDATE.
• lib/retry.py: RetryHandler.wait(err=RetryTimeout()) created one exception instance at definition time, shared by every call (traceback accumulates on reuse). It now defaults to None and a fresh RetryTimeout is raised. Also removed an obsolete Python 2 comment.

No behaviour change for callers passing explicit arguments.

Testing

Only py_compile was run: pytest/flake8 are not installed in my environment, so please rely on CI for the test suite.