Agentforce Employee Agent: 'Answer Questions with Knowledge' returns 0 results silently — Search Index Ready, content indexed and keyword-matching, SOSL works. 19 hypotheses tested. Backend AiRagFeaturePromptContent issue?

## Context
Building a POC Agentforce for Service on a production org (Enterprise Edition, Lightning Knowledge, Data Cloud enabled, Foundations active).
**Agent**: Employee Agent (`BotType = AgentforceEmployeeAgent`, `PlannerType = AiCopilot__ReAct`, `BotUserId = null` → runtime user = logged-in user, who is System Administrator with full Knowledge perms).
**Goal**: technician asks "How do I configure the VPN FortiClient?" → agent retrieves the relevant Knowledge article and answers.
## Problem
The action `Answer Questions with Knowledge` (`EmployeeCopilot__AnswerQuestionsWithKnowledge`, `invocationTarget = streamKnowledgeSearch`) **executes successfully but returns 0 results silently**:
```json
Output: {"knowledgeSummary": "Sorry, I can't find an answer based on the available articles."}

The same query via direct SOSL on Knowledge__kav returns the articles correctly — so it's NOT a Knowledge data problem, it's the RAG pipeline.

Event log input observed:

{
  "citationsEnabled": "true",
  "ragFeatureConfigId": "AiRagFeaturePromptContent_e3a977c6_c149_44c0_81fe_a6876f452fd2",
  "query": "Comment configurer le VPN FortiClient ?"
}

The ragFeatureConfigId GUID is the same since the Data Library was first created — even after delete + recreate of the library, Salesforce reuses the same backend record.

What's verified OK

• 4 Knowledge articles, PublishStatus=Online, Language=fr, IsVisibleInApp=true
• Articles are RecordType=Procedure with content in Procedure_Steps__c (10–33k chars each, real procedure steps)
• Data Library assigned to the agent (Knowledge type, default Data Space)
• Content Fields include Procedure_Steps__c and all Procedure_*__c
• Identifying Fields: Title + Summary
• Filter by Knowledge Data Categories: OFF
• Use Public Knowledge Articles: ON

Search Index state (via SOQL on DataSemanticSearch):

• RuntimeStatus = READY
• SearchType = Hybrid (semantic + keyword)
• IndexRefreshedOn recent (post-publish)
• 290 chunks in *_chunk__dlm, 13 contain "FortiClient" literally
• 290 vectors in *_index__dlm
• Max chunk length: 2104 chars (Procedure_Steps content confirmed indexed)
• SharingStatus = REGULAR

Things I tested (none fixed it)

1. Toggle Filter Data Categories OFF (was a forum-suggested cause)
2. Created Data Category Group, tagged all 4 articles, configured Data Category Visibility "All" on the user profile + Default Visibility
3. Topic language changed from en_US to fr (matches articles)
4. Topic instructions translated to French and customized (was the generic English SvcCopilotTmpl__GeneralFAQ template)
5. Activate/Deactivate cycle on the agent
6. Rebuild of the Search Index (SUBMITTED → IN_PROGRESS → READY)
7. Delete + recreate the Data Library → Salesforce reuses the same DataSemanticSearch GUID + same AiRagFeaturePromptContent_* GUID (singleton per data space? bug?)
8. Tried REST endpoints to query the index directly in v60/v62/v66: /ssot/queries/semantic-search, /ssot/queries/hybrid-search, /ssot/retrievers → all 404

Other relevant settings

• Knowledge.settings: enableKbStandardSharing = false (legacy sharing model)
• Knowledge.settings: defaultLanguage = fr (matches articles)
• Profile (System Admin): AllowViewKnowledge = true, EditKnowledge = true, ManageKnowledge = true
• Note: Einstein Agent license users in the org have UserPermissionsKnowledgeUser blocked by license (not used at runtime here since Employee Agent uses logged-in user — but mentioning for completeness)

Questions for the community

1. The AiRagFeaturePromptContent_* GUID — how do I inspect/debug its actual configuration? Is there a Setup UI, a Tooling API object, or a REST endpoint to retrieve its parameters (similarity threshold, top_k, default filters)?
2. Is it expected that delete + recreate of a Data Library reuses the same backend DataSemanticSearch and AiRagFeaturePromptContent? If yes, how does one force a clean reset of the backend RAG config?
3. Does the Data Cloud Hybrid retriever respect the legacy Knowledge sharing model (enableKbStandardSharing=false), the modern one, or its own independent permission layer?
4. What's the correct REST path for direct hybrid/semantic search against a DataSemanticSearch index (outside the agent context, for debugging)? /ssot/queries/semantic-search 404s in API v60-v66.
5. Has anyone else seen this exact symptom — Search Index Ready, content indexed and keyword-matching, agent returns 0 silently — and what was the fix?

Thanks for any pointers!