fix: fail fast when package manager is not installed (#360)

* fix: fail fast with clear error when package manager is not installed

When PMG shims intercept a package manager that isn't installed (e.g. a
clean JAMF-provisioned laptop where PMG is set up before dev tooling),
real-binary resolution failed with a generic "unknown" error and a
bug-report link, making it look like PMG itself had crashed.

Introduce a typed BinaryNotFoundError that exits with code 127 (standard
"command not found") and maps to a new PackageManagerNotFound error code
with an actionable message instead of the Unknown classification.

Co-authored-by: Cursor <cursoragent@cursor.com>

* fix: improve missing package manager help message

Use a concise, dynamic install hint instead of explaining PMG's
PATH forwarding internals.

Co-authored-by: Cursor <cursoragent@cursor.com>

* fix: mention PATH in missing package manager help text

Covers the common case where a package manager is installed but its bin
directory is not on PATH yet (e.g. after curl | bash install).

Co-authored-by: Cursor <cursoragent@cursor.com>

---------

Co-authored-by: Cursor <cursoragent@cursor.com>

internal/ui/exit_test.go

@@ -5,6 +5,7 @@ import (
 	"fmt"
 	"testing"
 
+	"github.com/safedep/pmg/internal/shim"
 	"github.com/stretchr/testify/assert"
 )
 
@@ -121,3 +122,21 @@ func TestClassifyExit(t *testing.T) {
 		assert.False(t, d.notice)
 	})
 }
+
+func TestExitFromCommandErrorExitCode(t *testing.T) {
+	t.Run("missing package manager uses exit code 127", func(t *testing.T) {
+		err := &shim.BinaryNotFoundError{Name: "bun"}
+
+		var ec exitCoder
+		assert.True(t, errors.As(err, &ec))
+		assert.Equal(t, 127, ec.ExitCode())
+	})
+
+	t.Run("wrapped missing package manager preserves exit code 127", func(t *testing.T) {
+		err := fmt.Errorf("execute: %w", &shim.BinaryNotFoundError{Name: "npm"})
+
+		var ec exitCoder
+		assert.True(t, errors.As(err, &ec))
+		assert.Equal(t, 127, ec.ExitCode())
+	})
+}