agents: merge main at the start of every iteration

Both agents read the protocol, their mission and the shared tooling from their
OWN checkout, and both work on topic branches -- so without an explicit sync
they follow whichever version of the rules existed when the branch started.

Found concretely: tools/audio-capture and two protocol revisions were on main
while the decoder worked for hours from a branch that had neither. The port had
merged on its own initiative and did have them, which is exactly the kind of
divergence nobody notices until the two disagree about what the rules say.

docs/agents/decoder-loop.md

@@ -13,6 +13,20 @@ behavioural and cannot be answered from a file, so you run the emulator.
 You do **not** build the port. If you find yourself writing GDScript or designing
 an export schema, stop and go back to the question you were answering.
 
+## Before anything else, every iteration: sync with `main`
+
+```bash
+git -C /work fetch origin && git -C /work merge --no-edit origin/main
+```
+
+You work on a topic branch, and you read the protocol, the mission and the
+shared tooling **from your own checkout** — so without this you are following
+whichever version of the rules existed when your branch started. That is not
+hypothetical: `tools/audio-capture` and two protocol revisions landed on `main`
+while one agent worked for hours from a branch that had neither.
+
+If the merge conflicts, resolve it, say so in your reply, and carry on.
+
 ## Read these first, every iteration
 
 1. `docs/agents/PROTOCOL.md` — how this team works. Non-negotiable.