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

README.md

@@ -118,8 +118,14 @@ that enforces the manifest before it leaves the host.
   upstream has *now* (fail-closed if unreachable). The agent's
   `~/.gitconfig` rewrites the real URL to the gate via `insteadOf`,
   so push, fetch, clone, and pull all route through. The agent
-  never sees the upstream credential. Brought up only when
+  never sees the upstream credential. If the upstream's hostname
-  `bottle.git` has entries. Design in `docs/prds/0008-git-gate.md`.
+  isn't resolvable from the gate container (e.g. a Tailscale-only
+  host whose public DNS points elsewhere), pin its IP via
+  `ExtraHosts: { "<hostname>": "<ip>" }` on the `bottle.git` entry —
+  the gate's `/etc/hosts` gets the override while the agent's
+  `insteadOf` rewrite still keys off the original hostname. Brought
+  up only when `bottle.git` has entries. Design in
+  `docs/prds/0008-git-gate.md`.
 
 When the agent exits, `cli.py` tears down every sidecar that was
 brought up and the two networks; nothing about a bottle persists

claude-bottle.example.json

@@ -19,14 +19,13 @@
         "GIT_AUTHOR_NAME": "Eric Diderich",
         "NODE_ENV":        "development"
       },
-      "ssh": [
+      "git": [
         {
-          "Host":         "gitea",
+          "Name":         "claude-bottle",
-          "Hostname":     "gitea.dideric.is",
+          "Upstream":     "ssh://git@gitea.dideric.is:30009/didericis/claude-bottle.git",
-          "User":         "git",
-          "Port":         30009,
           "IdentityFile": "/Users/didericis/.ssh/id_ed25519_gitea",
-          "KnownHostKey": "gitea.dideric.is ssh-ed25519 AAAA..."
+          "KnownHostKey": "ssh-ed25519 AAAA...",
+          "ExtraHosts":   { "gitea.dideric.is": "100.78.141.42" }
         }
       ],
       "egress": {

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,