FN-5653: inject active goal context into heartbeat and executor prompts

Align heartbeat and executor prompt assembly so both lanes receive the same active-goal context block.

- inject goalContext into heartbeat and executor buildPromptLayers inputs using buildGoalContextSection
- extend goal-context regression coverage with parity tests for heartbeat/executor injection and empty-goal behavior
- update executor test helpers for goal-store access and document the shared prompt-lane behavior in architecture/agents docs

Files changed:
 docs/agents.md                                     |  1 +
 docs/architecture.md                               |  1 +
 packages/engine/src/__tests__/executor-test-helpers.ts  |  3 +
 packages/engine/src/__tests__/goal-context-injection.test.ts   | 72 ++++++++++++++++++++++
 packages/engine/src/agent-heartbeat.ts             | 12 +++-
 packages/engine/src/executor.ts                    | 12 +++-
 packages/engine/src/prompt-layers.ts               |  9 ++-
 7 files changed, 107 insertions(+), 3 deletions(-)

Fusion-Task-Id: FN-5653
Fusion-Task-Lineage: d2670585-57f7-409d-bb11-cef3448267a6

packages/engine/src/__tests__/executor-test-helpers.ts

@@ -364,6 +364,9 @@ export function createMockStore() {
     listWorkflowSteps: vi.fn().mockResolvedValue([]),
     setPluginWorkflowStepTemplates: vi.fn(),
     appendAgentLog: vi.fn().mockResolvedValue(undefined),
+    getGoalStore: vi.fn().mockReturnValue({
+      listGoals: vi.fn().mockReturnValue([]),
+    }),
     getFusionDir: vi.fn().mockReturnValue("/tmp/test/.fusion"),
     clearStaleExecutionStartBranchReferences: vi.fn().mockReturnValue([]),
   };