fix(config): deterministic home-dir fallback in Path()

os.UserHomeDir()'s error was discarded; when it fails (home env var unset)
home became "", yielding a cwd-relative config path silently read/written
in an unexpected location. New homeDir() helper falls back to the platform
home env var (USERPROFILE/HOME), then os.TempDir(), so Path() is always
absolute. No signature change — callers untouched. Regression tests added.

Author: nixrajput

internal/config/path.go

@@ -15,7 +15,7 @@ func Path() string {
 	if v := os.Getenv("XDG_CONFIG_HOME"); v != "" {
 		return filepath.Join(v, "siphon", "config.yaml")
 	}
-	home, _ := os.UserHomeDir()
+	home := homeDir()
 	switch runtime.GOOS {
 	case "windows":
 		if v := os.Getenv("APPDATA"); v != "" {
@@ -26,3 +26,22 @@ func Path() string {
 		return filepath.Join(home, ".config", "siphon", "config.yaml")
 	}
 }
+
+// homeDir resolves the user's home directory deterministically. os.UserHomeDir
+// only fails when the platform's home env var is unset; if that happens we fall
+// back to that env var directly and finally to os.TempDir(), so the result is
+// always an absolute path — never "" (which would yield a cwd-relative config
+// path silently written/read in an unexpected location).
+func homeDir() string {
+	if h, err := os.UserHomeDir(); err == nil && h != "" {
+		return h
+	}
+	envVar := "HOME"
+	if runtime.GOOS == "windows" {
+		envVar = "USERPROFILE"
+	}
+	if h := os.Getenv(envVar); h != "" {
+		return h
+	}
+	return os.TempDir()
+}

internal/config/path_test.go

@@ -0,0 +1,35 @@
+package config
+
+import (
+	"path/filepath"
+	"testing"
+)
+
+// TestPath_AlwaysAbsolute is the regression for the ignored os.UserHomeDir
+// error: even when SIPHON_CONFIG_HOME / XDG_CONFIG_HOME are unset and home
+// resolution must fall back, Path() must return an absolute path — never a
+// cwd-relative one like ".config/siphon/config.yaml".
+func TestPath_AlwaysAbsolute(t *testing.T) {
+	t.Setenv("SIPHON_CONFIG_HOME", "")
+	t.Setenv("XDG_CONFIG_HOME", "")
+
+	got := Path()
+	if !filepath.IsAbs(got) {
+		t.Fatalf("Path() = %q; want an absolute path", got)
+	}
+}
+
+// TestHomeDir_FallsBackWhenHomeUnset verifies homeDir never returns "" even
+// when the platform home env var is cleared — it falls back to os.TempDir().
+func TestHomeDir_FallsBackWhenHomeUnset(t *testing.T) {
+	t.Setenv("HOME", "")
+	t.Setenv("USERPROFILE", "")
+
+	got := homeDir()
+	if got == "" {
+		t.Fatal("homeDir() = \"\"; want a non-empty absolute path")
+	}
+	if !filepath.IsAbs(got) {
+		t.Fatalf("homeDir() = %q; want an absolute path", got)
+	}
+}