feat: chat explorations saved as KB entries

[rajnavakoti]

Description

Save multi-turn Ask conversations as "exploration" entries in the KB. When a conversation uncovers new knowledge or connections, it can be promoted from a chat session to a persistent KB artifact.

Acceptance Criteria

• "Save as exploration" button in Ask page
• Exploration saved as a markdown file with conversation summary, key findings, and entity references
• Explorations browsable in a dedicated section
• Entities referenced in explorations get backlinks

Out of Scope

• Auto-extraction of entities from conversations
• Depends on #148 (chat persistence)