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.

internal/server/ui/ui.go

@@ -91,6 +91,7 @@ func New() (*Renderer, error) {
 		"templates/partials/host_row.html",
 		"templates/partials/toast.html",
 		"templates/partials/awaiting_agent.html",
+		"templates/partials/host_chrome.html",
 	}
 
 	pageEntries, err := fs.Glob(web.FS, "templates/pages/*.html")