Add Knowledge Base Feature to Tesks

Scoping (needs product sign-off before code — larger feature): a knowledge base linked to the Discord bot. Sketch: a tenant-scoped kb_article table (title, body markdown, tags), MCP/bot tools kb_add / kb_search / kb_get, surfacing in the bot ("what's our policy on X?") and injecting relevant articles into the triage prompt (like ProjectContextStore). Open questions: who authors/edits (UI vs bot), retrieval (FTS vs embeddings), and how it's surfaced. Recommend a short design issue before implementation.