Add --max-reflections CLI option (fixes #3865)

[Discotech17]

Fixes #3865

max_reflections was hardcoded to 3 in base_coder.py with no way
to configure it. This adds a --max-reflections CLI flag so users can
adjust it for complex tasks.

Changes:

• args.py: add --max-reflections argument (default: 3)
• base_coder.py: move max_reflections from class attribute to __init__ parameter
• main.py: pass args.max_reflections into Coder.create()

Also works via:

• Config file: max-reflections: 5 in .aider.conf.yml
• Env var: AIDER_MAX_REFLECTIONS=5

No behavior change for existing users (default remains 3).

[arnowaschk]

This is really helpful for local LLM setups!