P2R-02 slice 1: host-detail sub-tab skeleton

Extract header/vitals/sub-tabs into a host_chrome partial that every
host-detail tab page renders. Sources / Schedules / Repo go from
inert divs to real <a> links backed by stub pages that share the
chrome and a 'coming next' body — slices 2/3/4 fill them in.

Also re-establishes the version indicator (host_schedule_version vs
agent's applied_schedule_version) in the header.

Drops the legacy fat-schedule list/edit templates that referenced
fields removed by the P2 redesign (Manual / Paths / RetentionPolicy
on Schedule); the new templates land in slice 3.

web/templates/pages/host_sources.html

@@ -0,0 +1,13 @@
+{{define "title"}}{{.Title}}{{end}}
+
+{{define "content"}}
+{{template "host_chrome" .}}
+<div class="max-w-[1280px] mx-auto px-8 pb-14 pt-6">
+  <div class="empty-state">
+    <h3 class="text-base font-medium tracking-[-0.005em]">Sources tab — coming next.</h3>
+    <p class="text-pretty text-ink-mute text-[13px] mt-2 mx-auto max-w-[480px] leading-[1.65]">
+      The source-group editor lands in P2R-02 slice 2.
+    </p>
+  </div>
+</div>
+{{end}}