fix(query): workstream resolution in init.milestone-op and roadmap.analyze (#3196)

- initMilestoneOp now accepts and propagates the workstream parameter:
  relPlanningPath(workstream) replaces the hardcoded '.planning' dir,
  getMilestoneInfo gets workstream passed, extractCurrentMilestone gets
  workstream passed, archiveDir is derived from planningDir not root.

- resolveQueryRuntimeContext now reads .planning/active-workstream as a
  third-priority fallback after --ws flag and GSD_WORKSTREAM env var,
  completing the documented resolution chain for all query handlers.

Closes #3196

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

sdk/src/query/query-runtime-context.ts

@@ -1,3 +1,5 @@
+import { join } from 'node:path';
+import { readFileSync, existsSync } from 'node:fs';
 import { findProjectRoot } from './helpers.js';
 import { validateWorkstreamName } from '../workstream-utils.js';
 
@@ -11,6 +13,35 @@ export interface QueryRuntimeContext {
   ws?: string;
 }
 
+/**
+ * Read the active workstream from `.planning/active-workstream` file.
+ *
+ * Mirrors the logic in workstream.ts:getActiveWorkstream — returns null
+ * when the file is missing, empty, contains invalid characters, or names
+ * a workstream directory that doesn't exist on disk.
+ */
+function readActiveWorkstreamFile(projectDir: string): string | null {
+  const filePath = join(projectDir, '.planning', 'active-workstream');
+  try {
+    const name = readFileSync(filePath, 'utf-8').trim();
+    if (!name || !validateWorkstreamName(name)) return null;
+    const wsDir = join(projectDir, '.planning', 'workstreams', name);
+    if (!existsSync(wsDir)) return null;
+    return name;
+  } catch {
+    return null;
+  }
+}
+
+/**
+ * Resolve the runtime context for a query invocation.
+ *
+ * Workstream resolution priority:
+ *   1. `--ws <name>` flag (input.ws)
+ *   2. `GSD_WORKSTREAM` environment variable
+ *   3. `.planning/active-workstream` file
+ *   4. Root `.planning/` (no workstream)
+ */
 export function resolveQueryRuntimeContext(input: QueryRuntimeContextInput): QueryRuntimeContext {
   const projectDir = findProjectRoot(input.projectDir);
 
@@ -22,8 +53,13 @@ export function resolveQueryRuntimeContext(input: QueryRuntimeContextInput): Que
   }
 
   const envWs = process.env.GSD_WORKSTREAM;
+  if (envWs && validateWorkstreamName(envWs)) {
+    return { projectDir, ws: envWs };
+  }
+
+  const fileWs = readActiveWorkstreamFile(projectDir);
   return {
     projectDir,
-    ws: envWs && validateWorkstreamName(envWs) ? envWs : undefined,
+    ws: fileWs ?? undefined,
   };
 }