codebase-design: design-it-twice sub-agents can build on conflicting facts, and the comparison doesn't check them

**What I ran:** `/improve-codebase-architecture`, then design-it-twice on one candidate. Following DESIGN-IT-TWICE.md, I spawned three sub-agents, each with a different constraint: minimize the interface, design for concurrency first, and optimize for the most common caller. All three briefs pointed at the same file of facts gathered beforehand: the call sites, their guards and the library's behavior. **What happened:** one design rested on a library fact that contradicted that file. It assumed the ORM's conditional `UPDATE` returns the rows it changed, and used that to report which writes applied. The facts file said the update returns a row count, and the installed library source confirms it: `UpdateQuery._execute -> int`. The comparison step contrasts depth, locality and seam placement, and has no step that checks the facts each design depends on. The false premise went through that comparison unchallenged. I caught it only by reading the library source myself. **What I expected:** the comparison to separate design differences from fact differences. When the sub-agents disagree on design, that is the point of the fan-out. When a design states a fact that contradicts the brief or the source, that is an error, and the comparison should surface it before judging the designs. A shared facts file alone does not prevent this, because the sub-agent read the file and still asserted the opposite. - Harness: Claude Code - Model: Opus 5.5