port: pin Godot 4.7.2, and decide the MCP/skills question

The pin was 4.3, chosen without checking; 4.7.2 is the current stable (released
2026-08-18). Getting this right matters more here than usual because the project
diffs Godot screenshots against a reference renderer, so the engine version is
part of the measurement.

No Godot MCP server. Not because they are bad -- the mature ones need a LIVE
editor plus a WebSocket plugin and a Python server, which is a daemon, an editor
process and a second runtime added to an unattended loop. And their headline
feature, scene-tree introspection and node manipulation, is built for someone
hand-authoring scenes in the editor. This port GENERATES screens from exported
JSON: the agent writes a loader, not a scene tree, so the feature that justifies
the complexity does not apply. What it actually needs to verify itself -- run
headless, screenshot, diff against sylpheed-cli screen render -- is already a
bash job.

Third-party skill packs are the opposite trade: pure context, no runtime. Worth
revisiting, deliberately not installed now, because a skill is instructions
injected into an agent running with approvals disabled -- a supply-chain
decision, not a default -- and because P0/P1 need no advanced GDScript. The
agent may propose one, naming the milestone it unblocks, for a human to vendor
and review.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

docker/Dockerfile

@@ -38,7 +38,7 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
 # Pinned. An engine version bump changes rendering, and this project compares
 # screenshots against a reference renderer — so an upgrade must be a deliberate,
 # stated act rather than a silent drift.
-ARG GODOT_VERSION=4.3
+ARG GODOT_VERSION=4.7.2
 RUN cd /tmp \
     && curl -fsSLO "https://github.com/godotengine/godot/releases/download/${GODOT_VERSION}-stable/Godot_v${GODOT_VERSION}-stable_linux.x86_64.zip" \
     && unzip -q "Godot_v${GODOT_VERSION}-stable_linux.x86_64.zip" \