Keep joint_trigger on main-tree round-trips

[AngriestBird]

Bottom line

A focus in the main tree that carries a joint_trigger block loses it on export. The
extra-tree exporter keeps it; the main-tree one does not.

Why

focus_tree/parse.py:186-188 captures the block off any focus block, not just a
joint_focus:

jt = blocks.get("joint_trigger")
if jt:
    raw_rewards[(fid, "_joint_extra")] = f"joint_trigger = {{\n{jt}\n}}"

focus_tree/build.py:229 attaches it to every built focus regardless of tree.
focus_tree/codec.py:182-184 only emits it when include_joint_extra is true, and that
parameter defaults to False (codec.py:140).

export_focus_tree passes include_joint_extra=True (focus_tree/export.py:69-80).
export_main_tree does not pass it at all (focus_tree/export.py:219-228), so the block
is dropped with no error or warning.

AGENTS.md:48 states the rule this breaks: "Parse→export of an existing file must not
drop fields." No test covers it — tests/test_focus_tree_export_main.py has no
joint_trigger case, and tests/test_focus_tree_roundtrip.py only exercises
export_focus_tree.

Acceptance

• A main-tree focus with a joint_trigger block round-trips through export_main_tree
  unchanged.
• tests/test_focus_tree_export_main.py covers it.

BLUF