Craft config.ts caches: _configCache, _globalGitHubConfigCache, _configPathCache

src/config.ts holds three module-level caches: _configPathCache (found config file path — invariant across workspace selection, never cleared), _configCache (resolved CraftProjectConfig — cleared by setActiveWorkspace), _globalGitHubConfigCache (typed GitHubGlobalConfig | null | undefined: undefined = not yet computed, null = computed-and-not-found; cleared by setActiveWorkspace since GitHub config can differ per workspace). When adding a new workspace-dependent derived value, mirror this: add a module-level cache, clear it inside setActiveWorkspace(), and keep undefined/null distinct when 'not found' is itself a valid computed result (don't collapse both to one sentinel).