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.
