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.

docs/swarm/builder.md

@@ -127,7 +127,8 @@ commits nothing.
 swarm-controller sends the request. It renders `internal/meta`'s `flake.nix`
 with the same renderer hive-c0re uses (`hive-meta-flake`), all over `https`.
 Each custom agent that a hive places pins to its `agent-configs/<agent>`
-`main`. Hyperhive pins to one rev, a ratchet on the controller's own
+`main`; the controller omits and logs one whose repo has no commit on `main`, so it
+doesn't block the rest. Hyperhive pins to one rev, a ratchet on the controller's own
 running rev (`services.hyperhive.c0re.hyperhiveRev`, from `self.rev`):
 
 - the first commit pins hyperhive's forge `main`;