feat(FN-3302): restore Claude usage tracking with Fusion Anthropic auth sto

Restores Claude usage tracking across the dashboard by integrating with Fusion Anthropic auth storage, and adds detection and testing for the `model_context_window_exceeded` stop reason to handle context limit errors gracefully. The work spans planning logic, usage tracking, mission management UI, a

Fusion-Task-Id: FN-3302

packages/engine/src/__tests__/context-limit-detector.test.ts

@@ -88,6 +88,28 @@ describe("isContextLimitError", () => {
     expect(isContextLimitError("Context Window Exceeds limit")).toBe(true);
   });
 
+  // ── Anthropic model_context_window_exceeded stop_reason ──────────────
+
+  it("matches full pi-ai error 'Unhandled stop reason: model_context_window_exceeded'", () => {
+    expect(isContextLimitError("Unhandled stop reason: model_context_window_exceeded")).toBe(true);
+  });
+
+  it("matches standalone 'model_context_window_exceeded'", () => {
+    expect(isContextLimitError("model_context_window_exceeded")).toBe(true);
+  });
+
+  it("matches 'model_context_window_exceeded' case-insensitively", () => {
+    expect(isContextLimitError("Model_Context_Window_Exceeded")).toBe(true);
+  });
+
+  it("does NOT match 'model_context_window' without 'exceeded'", () => {
+    expect(isContextLimitError("model_context_window")).toBe(false);
+  });
+
+  it("does NOT match 'context_window' alone", () => {
+    expect(isContextLimitError("context_window")).toBe(false);
+  });
+
   // ── Negative matches: must NOT trigger ─────────────────────────────
 
   it("returns false for empty string", () => {

packages/engine/src/context-limit-detector.ts

@@ -42,6 +42,9 @@ const CONTEXT_OVERFLOW_PATTERNS: RegExp[] = [
   // Provider JSON error envelope variant: "context window exceeds limit (2013)"
   // Matches when "context window" and "exceeds" appear together (order-flexible)
   /context\s+window\s+exceeds/i,
+  // Anthropic API stop_reason: model_context_window_exceeded
+  // Surfaces as "Unhandled stop reason: model_context_window_exceeded" from pi-ai's mapStopReason()
+  /model_context_window_exceeded/i,
 ];
 
 /**