Chore: Add PIP variable to Makefile

- Defines a PIP variable to be used in the install target.
- This allows for more explicit control over which pip executable is used.
- Ensures consistency with how other tools like PYTHON and RUFF are defined.

Author: PythonicNinja

Makefile

@@ -8,6 +8,7 @@ VENV_ACTIVATE := source $(VENV_BIN)/activate
 DEFAULT_PYTHON := $(if $(wildcard $(VENV_BIN)/python),$(VENV_BIN)/python,)
 
 PYTHON ?= $(if $(DEFAULT_PYTHON),$(DEFAULT_PYTHON),$(UVX) python)
+PIP ?= $(if $(DEFAULT_PYTHON),$(VENV_BIN)/pip,$(UVX) pip)
 RUFF ?= $(UVX) ruff
 PYTEST ?= pytest -vv --cov=simple_agent --cov-report=term-missing
 
@@ -18,7 +19,7 @@ venv:
 	@echo "Activate with: $(VENV_ACTIVATE)"
 
 install:
-	$(PYTHON) -m pip install -r requirements.txt
+	$(PIP) install -r requirements.txt
 
 run:
 	$(PYTHON) main.py "$(PROMPT)"