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/path.go

@@ -1,6 +1,7 @@
 package shim
 
 import (
+	"errors"
 	"fmt"
 	"os"
 	"os/exec"
@@ -79,6 +80,9 @@ func ResolveRealBinary(name string) (string, error) {
 
 	resolved, err := exec.LookPath(name)
 	if err != nil {
+		if errors.Is(err, exec.ErrNotFound) {
+			return "", &BinaryNotFoundError{Name: name}
+		}
 		return "", fmt.Errorf("could not find %s in PATH (excluding pmg shims): %w", name, err)
 	}