leave an agent with no config commit out of internal/meta; read config heads from main

on deploy, one agent whose agent-configs repo has no commit on main is
skipped and logged instead of failing the whole internal/meta render, so the
other agents' pin moves go through. the controller reads each config head
from main, the branch hive-c0re compares against, not the repo's default
branch.

swarm-controller/src/forge/meta_repo.rs

@@ -17,9 +17,10 @@ impl MetaForge for Client {
         &self,
         owner: &str,
         repo: &str,
+        branch: Option<&str>,
         paths: &[&str],
     ) -> Result<Option<DefaultBranchFiles>> {
-        self.default_branch_files(owner, repo, paths).await
+        self.branch_files(owner, repo, branch, paths).await
     }
 
     async fn ensure_meta_repo(&self) -> Result<()> {