Merge origin/main into AI-Form-Fill

Resolution favours main for all conflicts (new orchestrator shape with
artifacts/file_names/conversation_history/resume_with, WorkflowOutcome
enum, RAG, ledger agent, deleted engine/config/.env.example).

Re-wires our three form-fill agents (FormAnalyserAgent, FormFillerAgent,
DocumentExtractorAgent) into the new app layout:
- contracts/__init__.py: adds form-fill exports to __all__
- api/dependencies.py: adds get_form_analyser_agent / get_form_filler_agent / get_document_extractor_agent
- api/app.py: instantiates the three agents in lifespan and registers form_fill_router
- api/routes/__init__.py: exports form_fill_router
- tests/test_stirling_api.py: imports and registers form-fill stubs
- tests/test_stirling_contracts.py: adds back KnowledgeUpdateResponse discriminator test

Form fill remains accessible via its own endpoints (/api/v1/form/ai/analyse,
/fill-batch, /extract). Not wired as an orchestrator delegate — following
main's pattern where orchestrator only routes pdf_edit/pdf_question/user_spec
/math_auditor.

Follow-ups still needed:
- Thread conversation_history into form-fill agent prompts
- Align form-fill response outcomes with WorkflowOutcome enum
- Decide whether engine should return ToolOperationStep plans (main's
  new pattern per #6116) or keep returning fill values directly

All 127 engine tests pass.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

engine/src/stirling/api/dependencies.py

@@ -12,6 +12,8 @@ from stirling.agents import (
     PdfQuestionAgent,
     UserSpecAgent,
 )
+from stirling.agents.ledger import MathAuditorAgent
+from stirling.rag import RagService
 from stirling.services import AppRuntime
 
 
@@ -39,6 +41,18 @@ def get_execution_planning_agent(request: Request) -> ExecutionPlanningAgent:
     return request.app.state.execution_planning_agent
 
 
+def get_rag_service(request: Request) -> RagService:
+    return request.app.state.runtime.rag_service
+
+
+def get_rag_embedding_model(request: Request) -> str:
+    return request.app.state.runtime.settings.rag_embedding_model
+
+
+def get_math_auditor_agent(request: Request) -> MathAuditorAgent:
+    return request.app.state.math_auditor_agent
+
+
 def get_form_analyser_agent(request: Request) -> FormAnalyserAgent:
     return request.app.state.form_analyser_agent