Add a Client that mirrors the CLI command tree

[jon-trust]

Adds a Python library entry point that mirrors the CLI:

from tmxcaliber import Client

api = Client()
api.me()
api.threatmodels.list(provider="aws")
api.threatmodels.threats("aws-s3", feature_class="S3.FC1")
api.compliance.mappings.list(framework="nist-800-53-r5")

What changes

• Client, exported from the package root along with the error classes. Every CLI command is an attribute path built at construction from contract.ROUTES, so a route the API adds is a method and a command together, with no route named in the library code. Each operation carries an inspect.Signature derived from its route, so help() and argument errors read like an ordinary function.
• Credentials resolve exactly as the CLI's do. api_key and api_url arguments override both, and when both are passed the credentials file is not read. A key that would reach a stored endpoint raises the CLI's note as a UserWarning.
• Incomplete answers raise IncompleteAnswer, which carries the rows that did arrive. allow_incomplete=True mirrors --allow-incomplete.
• lib/remote/operations.py now owns the route-to-request logic: path filling, filters, paging and incompleteness. run_api_command and Client both call it, and the CLI's behavior is unchanged.
• Settings.stored_endpoint_note owns the stored-endpoint wording for both surfaces. sends_environment_key_to_stored_endpoint is renamed sends_key_to_stored_endpoint, and it also covers a key passed as an argument.
• README: new "Use the API from Python" section.
• Version: bumped to 0.6.0, with uv.lock.

Testing

• New test/test_remote_api.py, which iterates over ROUTES rather than naming routes: every route is reachable at its command path, sends its own path and filters, and has a signature matching its CLI options. It also covers paging, @release, refused arguments, required filters, incompleteness, credential precedence, and the stored-endpoint warning.
• Full suite: 638 passed. ruff format --check, ruff check and mypy are clean. uv build and twine check pass.