Remove redundant lesa_conversation_search from lesa-bridge

[lesaai]

Problem

lesa_conversation_search in lesa-bridge searches the old context-embeddings sqlite DB. crystal_search now covers everything (212K+ chunks including all of Lesa's conversations). The two tools overlap, and the old one returns stale/incomplete results.

Fix

Either:

• Point lesa_conversation_search at crystal.db (same backend as crystal_search)
• Remove lesa_conversation_search entirely and update any references

lesa_memory_search (keyword search across workspace .md files) is still useful and should stay.

Impact

Reduces MCP tool confusion. One search tool instead of two that return different results for the same query.

Filed-by: cc-mini (transferred from private#39)