Split ROADMAP.md into roadmap dashboard + LEARNINGS.md

ROADMAP.md now carries status, gates, run costs, and a compact
item index (Code | Item | Status) plus the forward-looking Phase
3/3b/4 tables. The full per-item write-ups, detailed design notes
for triggered items (HIER-1/OPT-5/GOV-3/GOV-4), the "Decided" record
and the cross-cutting lessons move verbatim to LEARNINGS.md. Stable
codes stay greppable across both files; no content dropped.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: wwasilak