fix(Makefile): simplify PYTHON default for macOS make.

Makefile

@@ -1,35 +1,23 @@
 ROOT_DIR := $(shell pwd)
-
-# Prefer PYTHON= override. Else shebang of `mytoolkit` on PATH. Else python3.
-ifndef PYTHON
-CLI_BIN := $(shell command -v mytoolkit 2>/dev/null)
-ifneq ($(CLI_BIN),)
-PYTHON := $(shell head -1 "$(CLI_BIN)" 2>/dev/null | sed 's|^#!||')
-endif
-ifeq ($(strip $(PYTHON)),)
-PYTHON := python3
-endif
-endif
+PYTHON ?= python3
 
 .PHONY: help install uninstall sync-skill test
 
 help:
 	@echo "Usage: make [target]"
 	@echo ""
-	@echo "  install     Editable install into CLI Python + sync skill"
-	@echo "              (refuse GHA runner/_work trees; PYTHON=$(PYTHON))"
-	@echo "  sync-skill  Copy the bundled agent skill to ~/.agents/skills/mytoolkit/"
+	@echo "  install     Editable install + sync skill (PYTHON=$(PYTHON))"
+	@echo "  sync-skill  Copy bundled agent skill to ~/.agents/skills/mytoolkit/"
 	@echo "  test        Run the pytest suite"
 	@echo "  uninstall   Uninstall mytoolkit and remove shell completions"
 
-# Truth source for day-to-day work: xiaohe-agent/packages/mytoolkit (submodule).
-# Refuse ephemeral GHA runner checkouts so PATH CLI cannot silently point at _work.
+# Working tree: xiaohe-agent/packages/mytoolkit (submodule of apaam/mytoolkit).
+# Refuse ephemeral GHA runner checkouts (_work) so PATH CLI cannot point there.
 install:
 	@case "$(ROOT_DIR)" in \
 	  */_work/*|*/.xiaohe/actions-runner/*) \
 	    echo "ERROR: refuse install from GHA runner worktree: $(ROOT_DIR)" >&2; \
 	    echo "Use: cd <xiaohe-agent>/packages/mytoolkit && make install" >&2; \
-	    echo "  or: cd <xiaohe-agent> && make update / ./setup.sh" >&2; \
 	    exit 1 ;; \
 	esac
 	@echo "Installing editable mytoolkit"