Add "pre" and "post" handlers

[robbintt]

Add handlers to Aider to improve extensibility and take pressure off the existing feature backlog.

These handlers are all hooks. I kept the more general term so it is more extensible.

Handlers are appropriate for many use cases:

• Existing pre and post actions should be moved to handlers (see included autotest_handler.py handler)
• MCP and other calls can be added to a pre-handler without extensive code changes
• Potential for extension to any other desired entrypoint, such as tool calls analyzing the generation stream for early completion or cancellation

Example .aider.conf.yml section:

handlers:
- name: file-adder
  config:
    reflections: 6
    model: gemini/gemini-2.5-flash
- name: autotest
  config:
    test_cmd: "echo noop"

Loaded handlers are visible at startup, otherwise the line is not shown:

[robbintt]

Example of missing handler:

[tomjuggler]

I think this might be a great upgrade for Aider - as long as it is optional, why not?

[robbintt]

I think this might be a great upgrade for Aider - as long as it is optional, why not?

Yes, it's a noop until configured.