docs(user-guide): document saved decks export/import + decrypt tool

[WilfordGrimley]

Summary

• Merge-time wiki checklist item from PR PR-6 deck portability: export/import + standalone decrypt tool #242 (deck portability, merge commit 5ddf109): the wiki page covering saved decks needed to reflect the new Export/Import capability and standalone decrypt tool.
• No wiki page or docs/user-guide.md section previously described saved decks / My Decks at all (docs/features/saved-decks.md isn't in wiki-publish-map.json, so it doesn't publish to the wiki, and user-guide.md's existing "Saving and re-using a project" section covers local-folder/Drive only). Added a new sibling section with minimal framing (My Decks page, signed-in save) plus the Export/Import + standalone decrypt-saved-deck-export/ tool blurb, linking out to docs/features/saved-decks.md for the full design.
• No policy pages touched; no new content beyond this feature's user-visible surface (crypto model intentionally left out of the wiki, per CLAUDE.md's "link to docs/ for detail" rule).

Test plan

• python3 .github/scripts/docs_lint.py → docs-lint: clean.
• pytest .github/scripts/tests/test_publish_wiki_link_rewrite.py .github/scripts/tests/test_publish_readme.py → 12 passed, 28 subtests passed
• Dry-ran .github/scripts/publish_wiki.py against a scratch clone of the live wiki repo: new section renders in User-Guide.md, and the docs/features/saved-decks.md link correctly rewrites to an absolute GitHub blob URL (that page isn't wiki-published)
• git diff --stat confirms only docs/user-guide.md changed — no README region, no generated wiki page hand-edited

Open items

1. docs/features/saved-decks.md's continued exclusion from wiki-publish-map.json is a pre-existing gap (predates this PR, unrelated to PR-6 deck portability: export/import + standalone decrypt tool #242) — flagging, not fixing here, since publishing the full crypto design as its own wiki page is a separate editorial call for the owner.

Co-Authored-By: Claude Fable 5 noreply@anthropic.com