fix: make Makefile Python default portable

Default PYTHON to python3 instead of /opt/homebrew/bin/python3.12.
Override with PYTHON=python3.12 make setup when needed.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: 2 people

Makefile

@@ -1,6 +1,6 @@
 .PHONY: help setup dev test lint clean docker smoke doctor
 
-PYTHON ?= /opt/homebrew/bin/python3.12
+PYTHON ?= python3
 VENV   := .venv
 
 help: ## Show available targets