* add register_tools hookspec to allow plugins to register tools
* add @llm.Tool decorator that introspects a callables type signature
  and generates a JSON schema
* expose installed tools to Models via new `tools` Model property
* implement tool calling in the openai Model
* register a default `read_files` tool

Note: this pins pytest_httpx until broken tests are fixed in
  simonw#580
Note: this requires openai>=1.40.0
Note: this drops 3.8 since a number of dependencies no longer
  support it, and adds 3.13 support

…ore tool calls.

Introspecting callable type hints is the default fallback if no schema is provided.