RenderTemplate replaced tokens one key at a time over a StringBuilder, so a
token appearing inside an already-substituted value got substituted again on a
later pass. The prompt briefs only escaped this because their callers happen to
pass "tasks" last -- reordering the dictionary or adding a fourth token would
have started rewriting user-authored task descriptions, which after the enhance
phase carry file paths and config snippets.
Single-pass regex over the template instead; unknown tokens still pass through.