feat(sdk): add --ws flag for workstream-aware execution (#1884)

Add a --ws <name> CLI flag that routes all .planning/ paths to
.planning/workstreams/<name>/, enabling multi-workstream projects
without directory conflicts.

Changes:
- workstream-utils.ts: validateWorkstreamName() and relPlanningPath() helpers
- cli.ts: Parse --ws flag with input validation
- types.ts: Add workstream? to GSDOptions
- gsd-tools.ts: Inject --ws <name> into all gsd-tools.cjs invocations
- config.ts: Resolve workstream-aware config path with root fallback
- context-engine.ts: Constructor accepts workstream via positional param
- index.ts: GSD class propagates workstream to all subsystems
- ws-flag.test.ts: 22 tests covering all workstream functionality

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

sdk/src/context-engine.ts

@@ -25,6 +25,7 @@ import {
   DEFAULT_TRUNCATION_OPTIONS,
   type TruncationOptions,
 } from './context-truncation.js';
+import { relPlanningPath } from './workstream-utils.js';
 
 // ─── File manifest per phase ─────────────────────────────────────────────────
 
@@ -77,8 +78,8 @@ export class ContextEngine {
   private readonly logger?: GSDLogger;
   private readonly truncation: TruncationOptions;
 
-  constructor(projectDir: string, logger?: GSDLogger, truncation?: Partial<TruncationOptions>) {
-    this.planningDir = join(projectDir, '.planning');
+  constructor(projectDir: string, logger?: GSDLogger, truncation?: Partial<TruncationOptions>, workstream?: string) {
+    this.planningDir = join(projectDir, relPlanningPath(workstream));
     this.logger = logger;
     this.truncation = { ...DEFAULT_TRUNCATION_OPTIONS, ...truncation };
   }