port: make the Godot menu shell the agent's primary objective

Sets a new mission: boot the real disc through developer splash -> intro video
-> title -> main menu -> submenus in Godot 4, interactively, with no gameplay,
no 3D and no emulator.

docs/port/MISSION.md defines it -- eight gated milestones, each finished by an
ARTIFACT rather than by compiling, plus the Ready Room as an explicitly gated
stretch goal with a one-iteration probe that decides go/no-go. GP_READY_ROOM is
1106 entries with 6 recoverable names and is ISL-scripted, so it is either a
week or a quarter, and the agent must not start it on its own authority.

Architecture, per the user's decision: the Godot project is INDEPENDENT of the
Rust viewer and never reads a disc format. An offline Rust exporter converts the
disc into open formats; Godot reads only those. No GDExtension, no Rust in the
Godot project, and sylpheed-viewer is off limits -- it stays the human's
verification tool with its static-data rule intact.

docs/port/FORMAT.md specifies the open format, versioned, because modding is the
port's second goal and that makes the layout a deliverable rather than a temp
directory: JSON over XML (Godot parses JSON natively; its XMLParser is SAX),
names never hashes, provenance in every generated file, and unknowns listed
rather than guessed.

The discipline the whole thing rests on is the derived/authored split. `export/`
is regenerated wholesale and never hand-edited; `authored/` is hand-written and
survives a re-export. Three things this milestone needs are NOT on the disc in
any decoded form -- which button does what, paint order, and menu sound cues --
so they live in `authored/` with a stated `why`. Deleting an authored entry
because the exporter can now emit it IS the measure of progress.

`export/` is gitignored: it is generated from the user's own disc and this stays
a clean-room repo.

Container: adds a pinned Godot 4 (windowed under Xvfb for screenshots, plus a
headless wrapper). ffmpeg already carries libtheora, which is the video target --
Godot 4 plays only Ogg Theora natively and the disc's ADV.wmv is WMV3/WMA Pro.

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

docker/agent/Dockerfile

@@ -68,6 +68,26 @@ RUN for t in clang clang++ lld ld.lld llvm-ar llvm-ranlib llvm-nm clang-cpp; do
 # Python consumer to protect, so installing into it is the honest simple option.
 RUN pip3 install --no-cache-dir --break-system-packages duckdb
 
+# ── Godot 4 ──────────────────────────────────────────────────────────────────
+# The port's runtime (docs/port/MISSION.md). Two binaries, deliberately:
+#
+#   godot           the editor/windowed build, run under Xvfb, for screenshots
+#                   that can be diffed against `sylpheed-cli screen render`
+#   godot-headless  --headless, for script-only checks that need no display
+#
+# The official builds are self-contained binaries, not a package, so they are
+# fetched rather than apt-installed. Pinned: an engine version bump changes
+# rendering, and this project compares screenshots against a reference renderer.
+ARG GODOT_VERSION=4.3
+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${GODOT_VERSION}-stable_linux.x86_64.zip" \
+    && mv "Godot_v${GODOT_VERSION}-stable_linux.x86_64" /usr/local/bin/godot \
+    && chmod +x /usr/local/bin/godot \
+    && printf '#!/bin/sh\nexec /usr/local/bin/godot --headless "$@"\n' > /usr/local/bin/godot-headless \
+    && chmod +x /usr/local/bin/godot-headless \
+    && rm -f "Godot_v${GODOT_VERSION}-stable_linux.x86_64.zip"
+
 # ── Node + Claude Code ───────────────────────────────────────────────────────
 RUN curl -fsSL https://deb.nodesource.com/setup_22.x | bash - \
     && apt-get install -y --no-install-recommends nodejs \