prototype: capture the prototype, don't dispose of it

Reframe the artifact handling as capturing the prototype somewhere out
of main — a throwaway branch — with a context pointer to it left on the
implementation issue, instead of "disposing" of it. The main branch
still keeps only the validated decision.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

docs/engineering/prototype.md

@@ -35,7 +35,7 @@ Picking the wrong branch wastes the whole prototype, so the question comes first
 
 A finished prototype leaves two things. The **answer** — the verdict plus the question it settled — is what you capture durably (a commit message, an ADR, an issue). The **prototype itself is a primary source** — the runnable evidence the answer came from.
 
-The prototype doesn't belong in the main repo: no tests, no error handling, nothing to maintain. But that's not a reason to destroy it. Once the answer is captured, fold any validated decision into the real code, then move the prototype onto a throwaway branch linked from the relevant issue — never merged. The main branch stays clean; the raw exploration stays one click away for anyone who wants to re-run it. A prototype left rotting in the main branch has outlived its purpose — a prototype kept as a primary source on a side branch hasn't.
+The prototype doesn't belong in the main branch: no tests, no error handling, nothing to maintain. But that's not a reason to destroy it. Once the answer is captured, fold any validated decision into the real code, then capture the prototype on a throwaway branch — out of main, never merged — and leave a context pointer to it on the implementation issue. The main branch stays clean; the raw exploration stays one click away for anyone who wants to re-run it. A prototype left rotting in the main branch has outlived its purpose — a prototype captured as a primary source on a side branch hasn't.
 
 ## Where it fits