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.go

@@ -0,0 +1,19 @@
+package shim
+
+import "fmt"
+
+const commandNotFoundExitCode = 127
+
+// BinaryNotFoundError is returned when a package manager name resolves through
+// PMG shims but the real binary is absent from PATH (after shim dirs are stripped).
+type BinaryNotFoundError struct {
+	Name string
+}
+
+func (e *BinaryNotFoundError) Error() string {
+	return fmt.Sprintf("%s is not installed", e.Name)
+}
+
+func (e *BinaryNotFoundError) ExitCode() int {
+	return commandNotFoundExitCode
+}