fix(install): remove vestigial rigs/ directory creation

The rigs/ directory was created by gt install but never used by
gt rig add, which puts rigs at the town root. Rather than restructure
the entire codebase to use rigs/, we remove the unused directory.

Flat structure is fine - similar to Android top-level layout with
100+ subprojects. Rigs at root works well and avoids a breaking change.

Closes #74

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/cmd/install_integration_test.go

@@ -33,7 +33,6 @@ func TestInstallCreatesCorrectStructure(t *testing.T) {
 	// Verify directory structure
 	assertDirExists(t, hqPath, "HQ root")
 	assertDirExists(t, filepath.Join(hqPath, "mayor"), "mayor/")
-	assertDirExists(t, filepath.Join(hqPath, "rigs"), "rigs/")
 
 	// Verify mayor/town.json
 	townPath := filepath.Join(hqPath, "mayor", "town.json")