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/retrofit.md

@@ -51,7 +51,8 @@ file. Instead, help the user mine the right chat, then stop:
      from the desktop app; the output lands on disk either way).
    - In that resumed chat, paste the three extraction prompts from Part 3 Step 1 of
      `retrofit-playbook.md` (A — knowledge → AGENTS.md + the `ln -s AGENTS.md CLAUDE.md`
-     symlink; B — the `## Current state` section; C — the secrets sweep).
+     symlink; B — the `## Current state` section, plus seeding ROADMAP.md with any longer-term
+     backlog; C — the secrets sweep).
    - Then come back to a fresh session in this folder and run `/retrofit` again — you'll
      pick up here, at Phase 3.
 3. Stop. This phase ends the run when the gate isn't satisfied.