New `instance.support` (`AGNES_INSTANCE_SUPPORT` env override)
config field renders operator-authored HTML in a mint-accent
callout panel inside the welcome hero on /home, below the Overview
footnotes. Designed for a one-line invitation pointing at a chat
space, mailing list, or runbook so every user knows where to ask
for help.
- `get_instance_support()` helper mirrors `get_instance_overview()`
(env > yaml > "" resolution, `| safe` filter trust boundary).
- Wired into the home template context as `config.INSTANCE_SUPPORT`.
- Template renders the callout inside the welcome hero, after the
Overview footnotes block — empty yaml hides the block so the OSS
stays vendor-neutral.
- Registered in `_KNOWN_FIELDS["instance"]` so the field appears
in `/admin/server-config` as "Available but unset" even before
the operator populates it (discoverability for first-time setup).
- 4 new tests cover the gated render path, the hidden-when-unset
path, and independence from `instance.overview`.
Operators who want to fill the block via terraform write the body
to `modules/.../assets/support.html` in their infra repo and
include it in the startup.sh yaml heredoc — the OSS template
treats this as one more `| safe`-rendered field, no other plumbing
needed.
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