docs(git-gate): document ExtraHosts on bottle.git entries

- example manifest swaps the gitea-dev bottle from ssh: to git:
  and shows ExtraHosts pinning gitea.dideric.is to its Tailscale IP
- README's git-gate paragraph names the field and the case it
  solves (upstream resolvable on the host but not from the gate
  container's default DNS)
- PRD 0008's manifest-field bullet mentions the field for parity

docs/prds/0008-git-gate.md

@@ -83,7 +83,12 @@ for a declared upstream:
 - **Manifest field.** `bottle.git` — a list of git remotes the
   bottle is allowed to talk to, each with the credential the gate
   uses to push upstream. The agent gets no parallel `bottle.ssh`
-  entry for those upstreams.
+  entry for those upstreams. Each entry may also carry an
+  `ExtraHosts: { hostname: ip }` map, surfaced to the gate as
+  `--add-host` so the gate can resolve upstreams whose public DNS
+  doesn't point at the reachable IP (e.g. Tailscale-only hosts).
+  The agent-side `insteadOf` rewrite keys off the original hostname,
+  so the manifest's `Upstream` URL stays human-readable.
 - **Agent-side URL rewrite.** Provisioner emits `~/.gitconfig`
   with `[url "<gate-url>"] insteadOf = <real-url>` so every git
   operation against the declared upstream (push, fetch, clone,