chore: shared permission allowlist, screenshot script, gitignore

Add project .claude/settings.json (read-only cargo check/clippy + git diff
allowlist; personal settings.local.json stays gitignored). Add e2e/shots.mjs
(one-off mobile screenshot seeder). Ignore load-test run artifacts.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.claude/settings.json

@@ -0,0 +1,9 @@
+{
+  "permissions": {
+    "allow": [
+      "Bash(cargo check *)",
+      "Bash(cargo clippy *)",
+      "Bash(git --no-pager diff *)"
+    ]
+  }
+}