fix(site): anchor the bind-mount ignores, and commit platform.json

`data/` without a leading slash matches a directory of that name at any
depth, so it silently swallowed src/data/platform.json -- the single file
every page and checkFacts.mjs reads.

The working tree still had it, so `npm run verify` passed locally with all
14 facts green. CI cloned fresh and `astro check` failed on the missing
module. Anchored both patterns to the repository root.

Verified the way it should have been the first time: exported HEAD to a
clean directory, npm ci, and ran the checks there rather than in the tree
that was hiding the problem.

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

.gitignore

@@ -9,8 +9,14 @@ node_modules/
 # The bind mounts (PLAN.md §6, §7). Neither belongs in the repo: `brand/` is the
 # operator's override of the stock branding, and `data/` is the beta signup store.
 # `brand-default/` IS committed — it is baked into the image and must always be complete.
-brand/
-data/
+#
+# Both patterns are anchored with a leading slash, and that is load-bearing: an
+# unanchored `data/` matches a directory of that name at ANY depth, so it silently
+# swallowed `src/data/platform.json` — the one file the whole accuracy machinery reads.
+# The working tree still had it, so every local check passed; CI, cloning fresh, failed
+# on the missing module. Anchor anything meant to match only the repository root.
+/brand/
+/data/
 
 # Environment
 .env