Let pushes go by an HTTPS token or an SSH deploy key

The push credential is either a push-only account's token, handed to git
through a credential helper that names environment variables, never
values, or a deploy key in a short-lived ssh-agent. The remote URL
follows the credential. Configured as gitea.push_user and push_token, or
gitea.push_key.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PPQ14L3LikyefCMFBqEbKE

docs/Architecture.md

@@ -674,9 +674,10 @@ Installing on any other host is dangerous.
   fresh and this one resumes from its branch.
 - **Publish only the work:** what the agent changed, counted from where the
   milestone started, never build artifacts; anything left out is reported.
-- **Pushes run on the build VM** with a key the model's commands never receive
-  (L12); PRs are opened from the agent's container with its API token, which never
-  reaches a build VM. Gitea is the only copy that has to survive.
+- **Pushes run on the build VM** with a push credential (a deploy key, or a
+  push-only account's token) the model's commands never receive (L12); PRs are
+  opened from the agent's container with its API token, which never reaches a
+  build VM. Gitea is the only copy that has to survive.
 - **Publishing is idempotent** (L8). A milestone with no changes is done if it wasn't
   expected to change code; otherwise it's raised with the operator.
 - **The agent never owns or administers a repository:** whoever does can turn
@@ -727,7 +728,7 @@ execution:
 
 **Secrets are namespaced by project.** A build VM's commands receive only their
 project's, and each secret's configuration says whether the model's commands may
-see it or only harness operations use it (the push key). Anything the model's
+see it or only harness operations use it (the push credential). Anything the model's
 commands can see must be scoped for testing, never production.
 
 **Provisioning:** build VMs come from a template via Terraform; eventually at