fix(#1913): derive workstream progress status from shipped signals (#1916)

workstream progress trusted the mutable STATE.md `Status` field, so a
shipped/archived milestone whose field was left at `executing` was reported
as executing — a stale hand-maintained field became the source of truth
instead of the authoritative archive/tag/ROADMAP signals.

Derive status in the inventory builder from a milestoneShipped signal
(archived milestone snapshot under milestones/, or a SHIPPED marker in the
workstream ROADMAP), collected by inspectWorkstream. The inventory now
reports `status_source` (field|derived) and `status_conflict` (true when
the derived value disagrees with the stale field), and a shipped workstream
is never reported executing.

- src/workstream-inventory-builder.cts: milestoneShipped input + status_source/status_conflict outputs + derivation
- src/workstream-inventory.cts: workstreamMilestoneShipped() signal detector wired into inspectWorkstream
- tests/workstream-inventory.test.cjs: regression (builder unit + inspectWorkstream integration + negative)

Closes #1913

src/workstream-inventory.cts

@@ -84,6 +84,30 @@ function readStateProjection(statePath: string): StateProjection {
   }
 }
 
+/**
+ * #1913: detect an authoritative shipped signal for a workstream so the
+ * inventory status is never trusted from the mutable STATE.md `Status` field
+ * alone. Returns true when EITHER an archived milestone snapshot is present
+ * under `<planningBase>/milestones/` OR the workstream ROADMAP carries a
+ * SHIPPED marker — both are hard to desync, unlike the hand-maintained field.
+ */
+function workstreamMilestoneShipped(roadmapPath: string, planningBase: string): boolean {
+  try {
+    const milestonesDir = path.join(planningBase, 'milestones');
+    for (const entry of fs.readdirSync(milestonesDir, { withFileTypes: true })) {
+      if (entry.isFile() && /-ROADMAP\.md$/i.test(entry.name)) return true;
+    }
+  } catch {
+    /* no milestones archive dir */
+  }
+  try {
+    if (/SHIPPED/i.test(fs.readFileSync(roadmapPath, 'utf-8'))) return true;
+  } catch {
+    /* no roadmap */
+  }
+  return false;
+}
+
 function sortWorkstreamInventories(inventories: WorkstreamInventory[], activeWorkstreamName: string | null): WorkstreamInventory[] {
   return [...inventories].sort((a, b) => {
     const aActive = a.name === activeWorkstreamName ? 1 : 0;
@@ -123,6 +147,7 @@ function inspectWorkstream(cwd: string, name: string, options: InspectWorkstream
       state: fs.existsSync(p.state),
       requirements: fs.existsSync(p.requirements),
     },
+    milestoneShipped: workstreamMilestoneShipped(p.roadmap, p.planning),
   });
 }