fix(install): Makefile uses go install to match docs (#851)

* fix(install): use go install to match docs

Makefile install target now uses 'go install' instead of cp to
~/.local/bin, aligning with documented installation method and
GOPATH/GOBIN conventions.

Closes: hq-93c
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

* fix(beads): set BEADS_DIR for config

* test(config,rig): update startup and bd stubs

---------

Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

internal/beads/beads_types.go

@@ -107,6 +107,7 @@ func EnsureCustomTypes(beadsDir string) error {
 	typesList := strings.Join(constants.BeadsCustomTypesList(), ",")
 	cmd := exec.Command("bd", "config", "set", "types.custom", typesList)
 	cmd.Dir = beadsDir
+	cmd.Env = append(os.Environ(), "BEADS_DIR="+beadsDir)
 	if output, err := cmd.CombinedOutput(); err != nil {
 		return fmt.Errorf("configure custom types in %s: %s: %w",
 			beadsDir, strings.TrimSpace(string(output)), err)