fix: reset data discovery tab session state on hypothesis change

[DavidJBianco]

Motivation

• The Data Discovery tab attempted to clear session keys named "Data Sources_document"/"Data Sources_messages" while the chat UI derives its session keys from doc_title="Discovery" (i.e., "Discovery_document"/"Discovery_messages"), which left stale discovery output and chat history when the hypothesis changed.

Description

• Update the hypothesis-change reset block in peak_assistant/streamlit/app.py to delete "Discovery_document" and "Discovery_messages" instead of the incorrect "Data Sources_*" keys so the tab clears its actual Streamlit session state.

Testing

• Verified the updated reset block and key usage with a code search (rg) which showed the corrected key names in the data_discovery_tab block.
• Successfully compiled the modified file with python -m compileall -q peak_assistant/streamlit/app.py with no syntax errors.

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Codex Task

[DavidJBianco]

Merged into dev via #84 (batch-a-quick-fixes, commit 90ee5d5).