MCP: fetch a single thought or insight by UUID

AI agents can now retrieve a single thought (get_thought) or insight (get_insight) directly by UUID — no search round-trip needed. This closes the gap where an agent holding an id (from a prior search result, a stored reference, or the source_thought_id backref left by promote_thought) had no way to pull the full record. get_insight also returns a version history summary so agents know how many revisions an insight has and what the latest change note says.