chore: honor XDG_DATA_HOME in Makefile install path

gh resolves its data dir from XDG_DATA_HOME (falling back to
~/.local/share/gh), so the hardcoded path put 'make install' in the
wrong place for anyone who sets XDG_DATA_HOME. Honor it, keeping the
documented default when unset.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: nodeselector

Makefile

@@ -1,6 +1,9 @@
 BIN := gh-actions-pin
 EXT_NAME := gh-actions-pin
-EXT_DIR := $(HOME)/.local/share/gh/extensions/$(EXT_NAME)
+# Honor XDG_DATA_HOME so this matches where gh actually resolves its data
+# dir; fall back to the documented default when it's unset.
+XDG_DATA_HOME ?= $(HOME)/.local/share
+EXT_DIR := $(XDG_DATA_HOME)/gh/extensions/$(EXT_NAME)
 
 .PHONY: build test install reinstall uninstall