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/runner/execute.go

@@ -2,6 +2,7 @@ package runner
 
 import (
 	"context"
+	"errors"
 	"fmt"
 	"io"
 	"os"
@@ -85,6 +86,10 @@ func ExecuteWithOptions(ctx context.Context, pc *packagemanager.ParsedCommand, o
 
 	realBinary, err := shim.ResolveRealBinary(pc.Command.Exe)
 	if err != nil {
+		var notFound *shim.BinaryNotFoundError
+		if errors.As(err, &notFound) {
+			return notFound
+		}
 		return fmt.Errorf("failed to resolve real %s binary: %w", pc.Command.Exe, err)
 	}