Draft Gall docs skeleton

pkg/arvo/neo/cod/std/src/fil/gall.md

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+# Gall
+
+Gall is Arvo's userspace software vane. Shrubbery interfaces with Gall by accepting subscription updates, {what else?}.
+
+## Types
+
+???
+
+{peer could be subscription, peek is probably a scry?}
+
+{what is `=<  gall` storing?}
+
+```
+++  gall
+  =<  gall
+  |%
+  +$  gall
+    $:  peer=(jug pith pith)
+        peek=(map pith peek:^^gall)
+    ==
+  --
+```
+
+The Gall shane takes four requests and only gives one response, a `(pair pith sign)` for subscription updates from agents.
+
+```
+++  gall
+  |%
+  +$  req
+    $%  [%peek p=@dr]
+        [%keep ~]
+        [%reap ~]
+        [%peer ~]
+    ==
+  +$  peek  [src=(set pith) refresh=@dr]
+  +$  sign  sign:agent:^gall
+  +$  res   (pair pith sign)
+  --
+```
+
+## Tasks
+
+### `%peek`
+
+???
+
+### `%keep`
+
+???
+
+### `%reap`
+
+???
+
+### `%peer`
+
+???
+
+## Gifts
+
+### `(pair pith sign)`
+
+Subscription update from a Gall agent, in practice usually `%facts`. The `pith` corresponds to the [`wire`](https://docs.urbit.org/courses/app-school/types#wire) and the `sign` the [`sign:agent:gall`](https://docs.urbit.org/system/kernel/gall/reference/data-types#signagent).