feat(agents): surface KB source citations in RAG responses

[petechentw]

Summary

Closes #9331

When an agent answers a question using the Knowledge Base (RAG), the response now appends a `Sources:` block listing the original documents the answer was drawn from — with clickable links back to the raw files.

This addresses all three points raised in the issue:

• The original file name (e.g. `How to use localai.pdf`) is included as a citation
• Citations appear at the end of the answer under a `Sources:` section
• Each file name is a clickable link that opens the original document

Changes

• `core/services/agents/citations.go` (new) — thread-safe citation collector, `AppendKBCitations` to build a markdown Sources block, raw-file URL builder, and markdown link escaping
• `core/services/agents/knowledge.go` — `KBAutoSearchPrompt` now returns a `KBSearchContext` (prompt + structured citations) instead of a plain string; adds `KBCitation`, `KBSearchContext`, and `KBCitationCollector` types; fixes URL encoding with `url.PathEscape` and `url.Values`
• `core/services/agents/executor.go` — wires citation collection through the full chat flow; appends citations to the final response while saving the citation-free version to memory so citations don't pollute long-term KB storage
• `core/services/agentpool/agent_pool.go` — integrates citation appending into the local chat path; falls back to a fresh auto-search if no citations were collected from tool calls

Example output

```
The quarterly revenue increased by 12% year-over-year...

Sources:
[1] Q3-report.pdf
[2] How to use localai.pdf
```

Notes

• Duplicate citations are deduplicated per source document (many chunks from the same file appear only once)
• The clean response without citations is saved to long-term memory to avoid polluting future KB searches
• Collection names and entry keys with special characters are properly URL-encoded
• Citations are collected from both auto-search results and `search_memory` tool calls during multi-step reasoning

[mudler]

nice! thanks @petechentw !