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/shim/binary_not_found_test.go

@@ -0,0 +1,14 @@
+package shim
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestBinaryNotFoundError(t *testing.T) {
+	err := &BinaryNotFoundError{Name: "bun"}
+
+	assert.Equal(t, "bun is not installed", err.Error())
+	assert.Equal(t, commandNotFoundExitCode, err.ExitCode())
+}