feat(mavweb): /ecosystem page consuming Nexus/Praxis/Hexis + shell fixes

Add a read-only /ecosystem page that consumes the sibling services'
JSON APIs (Nexus entities, Praxis attention, Hexis capabilities),
fetched concurrently with honest per-panel error states. Siblings stay
headless — mavweb is their human surface (arch §16). Wired via mavweb
-nexus/-praxis/-hexis flags; mavweb joins the ecosystem compose network.

Fix mobile horizontal overflow across all pages: .content is a flex
child with default min-width:auto, so it refused to shrink below the
tables' intrinsic width. min-width:0 lets wide tables pan inside .scroll
instead of dragging the page sideways. Verified via CDP geometry check
(scrollWidth === clientWidth at 430px).

Also includes in-progress Ethos UI redesign, ecosystem deploy compose,
and planning docs.

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

cmd/mavweb/trace.html

@@ -1,6 +1,6 @@
 {{template "shellTop" "trace"}}
 <h1>Rule Trace</h1>
-<div class=hint style=margin-bottom:var(--ethos-space-4)>{{.Now | ago}} — winner: <strong>{{if .Winner}}{{.Winner}}{{else}}nothing fired{{end}}</strong></div>
+<div class="hint mb-4">{{.Now | ago}} — winner: <strong>{{if .Winner}}{{.Winner}}{{else}}nothing fired{{end}}</strong></div>
 <div class=scroll><table class=mono>
 <tr><th>rule<th>sev<th>predicate<th>gate<th>blocked by<th>detail<th>selected<th>lost to</tr>
 {{range .Rules}}<tr>