fix: resolve desktop KB prompt self-contradiction on chunk_ids

The citations fix (cacb27e0) added a "Chunk citations in your prose"
section to system_prompt_desktop.md telling the KB subagent to always
leave `evidence.chunk_ids` null and emit no `[citation:...]` markers in
desktop mode, but left the pre-existing line declaring that
`chunk_ids` apply to `<priority_documents>` hits. The two rules
contradicted each other; the model picked one per turn.

Strike the stale conditional clause and point at the dedicated section
as the single source of truth. Matches the parallel line in
system_prompt_cloud.md and the already-consistent
system_prompt_readonly_desktop.md.

surfsense_backend/app/agents/multi_agent_chat/subagents/builtins/knowledge_base/system_prompt_desktop.md

@@ -33,7 +33,7 @@ Map outcomes to your `status`:
 - Any other `"Error: …"` → `status=error` and relay the tool's message verbatim as `next_step`.
 - HITL rejection → `status=blocked` with `next_step="User declined this filesystem action. Do not retry."`.
 
-You construct the structured `evidence` fields from your own knowledge of what you called and what you observed — the tools do not return them. `chunk_ids` apply only to `<priority_documents>` hits; for local-file operations leave them `null`. Never report values you did not actually see.
+You construct the structured `evidence` fields from your own knowledge of what you called and what you observed — the tools do not return them. Never report values you did not actually see. (`chunk_ids` is always `null` in desktop mode — see "Chunk citations in your prose" below.)
 
 ## Chunk citations in your prose