port: gitignore the export tree, which MISSION already said was ignored

`export/` is what `crates/sylpheed-export` writes and what
`ExportTree.locate()` reads. It was **not** gitignored. `data/base/` was --
the name MODDING.md gives the same tree, and a directory nothing writes.

So the live output directory was tracked while MISSION §4 states, as the
hardest rule in this project, that it 'is generated from the user's own disc
and is gitignored'. A `git add -A` in a container that had run the exporter
would have committed the disc: 16 screens of sprite PNGs and two transcoded
.ogv reels.

Nothing was committed -- this repository's history is clean, and a fresh
container reproduces the hazard rather than inheriting it, because the export
tree is regenerated and never cloned.

Both names are now ignored, so renaming the tree to match MODDING.md later
cannot silently re-open this.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Evuhbt8pxKJEUvwfniwYkU

.gitignore

@@ -25,6 +25,14 @@ __pycache__/
 # ── The port ────────────────────────────────────────────────────────────────
 # Generated from the user's own disc. This repo stays clean-room: code, schemas,
 # authored mappings and documentation only -- never game content.
+#
+# BOTH names are ignored on purpose. `export/` is what the exporter writes and
+# what `ExportTree.locate()` reads today; `data/base/` is the name MODDING.md
+# gives that same tree. Only one of them existed here, and it was the one
+# nothing writes -- so the live output directory was tracked while MISSION §4
+# said it was ignored. Ignoring both means renaming the tree to match the docs
+# cannot silently start committing the disc.
+/export/
 /data/base/
 # Transient inter-agent files. Deliberately outside history: they are working
 # artefacts with provenance in their manifest, not results.