hive-c0re: drop the remaining "mirror" wording for the removed push

The reseed bail text, the `ensure_all` doc, the forge module doc and a
rebuild comment still described hive-c0re mirroring the applied config to
`agent-configs/<name>`. State what the code does now; scope the
config-changes doc to swarm-created agents and say what a legacy agent
whose repo has no `main` gets.

Refs #4851

hive-c0re/src/lifecycle/mod.rs

@@ -295,8 +295,8 @@ pub async fn prepare_rebuild_dirs(name: &str, paths: &AgentPaths) -> Result<()>
     {
         bail!(
             "applied repo at {} is missing its .git directory and could not be reseeded from \
-             the forge (agent-configs/{name} unreachable, not yet mirrored, or absent); \
-             destroy --purge and re-spawn this agent.",
+             the forge (agent-configs/{name} unreachable, has no `main` — never merged or \
+             never seeded — or absent); destroy --purge and re-spawn this agent.",
             paths.applied.display()
         );
     }