Flesh out RAG system (#6197)

# Description of Changes
Flesh out the RAG system and connect it to the PDF Question Agent so it
can respond to questions about PDFs of an extremely large size.

I'd expect lots more work will need to be done to finish off the RAG
system to really be what we need, but this should be a reasonable start
which will let us connect it to tools and have the ingestion mostly
handled automatically. I'm leaving file deletion and proper file ID
management to be done in a future PR. We also need to consider whether
all tools should retrieve content exclusively via RAG, or whether it's
beneficial to have tools sometimes fetch the direct content and other
times fetch it from RAG.

A diagram of the expected interaction is as follows:

```mermaid
sequenceDiagram
    autonumber
    actor U as User
    participant FE as Frontend<br/>(ChatPanel)
    participant J as Java<br/>(AiWorkflowService)
    participant O as Engine:<br/>OrchestratorAgent
    participant QA as Engine:<br/>PdfQuestionAgent
    participant RAG as Engine:<br/>RagService + SqliteVecStore
    participant V as VoyageAI<br/>(embeddings)
    participant L as LLM<br/>(Claude / etc.)

    U->>FE: types "Summarise this PDF"<br/>(PDF already uploaded)
    FE->>J: POST /api/v1/ai/orchestrate/stream<br/>multipart: fileInputs[], userMessage
    Note over J: ByteHashFileIdStrategy<br/>id = sha256(bytes)[:16]
    J->>O: POST /api/v1/orchestrator<br/>{ files:[{id,name}], userMessage }

    O->>L: route via fast model
    L-->>O: delegate_pdf_question
    O->>QA: PdfQuestionRequest

    loop for each file
        QA->>RAG: has_collection(file.id)
        RAG-->>QA: false
    end
    QA-->>O: NeedIngestResponse(files_to_ingest)
    O-->>J: { outcome:"need_ingest", filesToIngest:[...] }

    Note over J: onNeedIngest
    loop per file
        J->>J: PDFBox: extract page text
        J->>O: POST /api/v1/rag/documents<br/>(long-running timeout)
        O->>RAG: chunk + stage documents
        O->>V: embed_documents (batches of 256)
        V-->>O: embeddings
        O->>RAG: add_documents
        O-->>J: { chunks_indexed: N }
    end

    Note over J: retry with resumeWith=pdf_question
    J->>O: POST /api/v1/orchestrator
    Note over O: fast-path to PdfQuestionAgent

    O->>QA: PdfQuestionRequest
    Note over QA: build RagCapability<br/>pinned to file IDs
    QA->>L: run(prompt) with search_knowledge tool

    loop up to max_searches
        L->>QA: search_knowledge(query)
        QA->>V: embed_query
        V-->>QA: query vector
        QA->>RAG: search(vector, collections=[file.id])
        RAG-->>QA: top-k chunks
        QA-->>L: formatted chunks
    end

    Note over QA: once budget spent,<br/>prepare() hides the tool
    L-->>QA: PdfQuestionAnswerResponse
    QA-->>O: answer
    O-->>J: { outcome:"answer", answer, evidence }
    J-->>FE: SSE "result"
    FE->>U: assistant bubble
```

engine/tests/agents/test_pdf_review.py

@@ -20,9 +20,9 @@ from stirling.agents.pdf_review import (
     _LocalisedComment,
     _LocalisedVerdict,
 )
-from stirling.contracts import EditPlanResponse, OrchestratorRequest, SupportedCapability
+from stirling.contracts import AiFile, EditPlanResponse, OrchestratorRequest, SupportedCapability
 from stirling.contracts.ledger import Discrepancy, DiscrepancyKind, Severity, Verdict
-from stirling.models import ToolEndpoint
+from stirling.models import FileId, ToolEndpoint
 from stirling.models.agent_tool_models import AgentToolId, PdfCommentAgentParams
 from stirling.services.runtime import AppRuntime
 
@@ -144,7 +144,10 @@ async def test_orchestrate_classifier_true_emits_math_audit_plan(runtime: AppRun
     """First turn — when the math-intent classifier says yes, emit a one-step plan
     calling the math auditor with resume_with=PDF_REVIEW."""
     agent = PdfReviewAgent(runtime)
-    request = OrchestratorRequest(user_message="vérifie les totaux", file_names=["report.pdf"])
+    request = OrchestratorRequest(
+        user_message="vérifie les totaux",
+        files=[AiFile(id=FileId("report-id"), name="report.pdf")],
+    )
 
     with patch.object(agent._math_intent_classifier, "classify", AsyncMock(return_value=True)):
         response = await agent.orchestrate(request)
@@ -161,7 +164,7 @@ async def test_orchestrate_classifier_false_routes_to_pdf_comment_agent(runtime:
     agent = PdfReviewAgent(runtime)
     request = OrchestratorRequest(
         user_message="review the invoices for ambiguous wording",
-        file_names=["contract.pdf"],
+        files=[AiFile(id=FileId("contract-id"), name="contract.pdf")],
     )
 
     with patch.object(agent._math_intent_classifier, "classify", AsyncMock(return_value=False)):
@@ -187,7 +190,7 @@ async def test_orchestrate_resume_uses_verdict_without_calling_classifier(
     verdict = _make_verdict([_discrepancy(page=0, stated="$100")])
     request = OrchestratorRequest(
         user_message="flag math errors",
-        file_names=["report.pdf"],
+        files=[AiFile(id=FileId("report-id"), name="report.pdf")],
         artifacts=[MathAuditorToolReportArtifact(report=verdict)],
     )
     canned = _LocalisedVerdict(