Wire ROADMAP.md through the standards docs

ROADMAP.md was named as a destination in two places but never created,
indexed, or read back. Add the convention to how-i-work.md, note it after
the README decision tree and in the loads-when table, give it a row in the
portability layer table, make the retrofit prompt create it, and have
handoff seed it and pull from it when Current state's next steps run thin.

guides/handoff.md

@@ -35,8 +35,12 @@ and why. The user may provide optional focus notes; weave them in where relevant
   - Next steps in priority order
   - Open questions, risks, or anything the user flagged that wasn't resolved
   - Test/build status if worth knowing
-- Anything longer-term goes in ROADMAP.md, not here. If Current state is accumulating
-  history, prune it — history lives in the git log.
+- Anything longer-term than the next steps goes in `ROADMAP.md` at the repo root, not here
+  — create it if it doesn't exist yet and there's backlog to record. If Current state is
+  accumulating history, prune it: finished narrative to the git log, still-real longer-term
+  items to ROADMAP.md.
+- Keep the two in sync: when the next steps here run thin, pull the next item(s) up from
+  ROADMAP.md so a fresh session always finds concrete next steps in Current state.
 
 ## 4. Final report