Clean up legacy 0.3.5 scaffolding and standalone-mode artifacts

- Removes start9/0.4/ — the StartOS 0.3.5-style YAML manifest folder,
  superseded by the TypeScript-based startos/ package.
- Removes pre-StartOS standalone-mode artifacts: setup.sh, create-app.sh,
  Start Summarizer.command, build-guide-pdf.py, GET-STARTED.md/.pdf.
- Removes conversion-era design docs (START9_PACKAGING_GAMEPLAN.md,
  UPGRADE-DESIGN.md, KEYSAT_INTEGRATION.md). Recoverable from history if
  needed.
- Tightens .gitignore: untracks node_modules, javascript/ build output,
  *.s9pk, history/ user data, cookies.txt, library export, .env, .DS_Store,
  and .claude/ worktree state. Files remain on disk; just removed from
  the index.

.gitignore

@@ -5,3 +5,27 @@
 # prompt. Written by Claude after code changes, consumed + deleted on the
 # next `make deploy` / `make bump`.
 .release-notes-pending.txt
+
+# OS
+.DS_Store
+
+# Node
+node_modules/
+
+# Build artifacts (regenerated by `make` / `npm run build`)
+javascript/
+*.s9pk
+image.tar
+
+# Runtime / user data — must never be committed
+history/
+cookies.txt
+*.txt.bak
+youtube-summarizer-library-export*.json
+ytdlp-cache/
+
+# Local dev secrets
+.env
+
+# Claude Code state (worktrees, plans, etc.)
+.claude/