Feature: search_knowledge should support filtering by stack

[MidOSresearch]

Problem

When calling search_knowledge("authentication patterns"), results come from all stacks (React, Django, NestJS, etc.). Sometimes I only want results for a specific stack.

Proposed Solution

Add an optional stack parameter:

search_knowledge("authentication patterns", stack="fastapi")

Similar to how list_skills already supports stack filtering.

Current Workaround

Using hybrid_search with stack name in the query works but is less precise.