Add sample guide, sample exports, and enriched build report

- examples/: sample guide store, .sfgz share archive, and exported
  output in all nine formats (JSON/MD/HTML×2/PDF/GIF/images/DOCX/PPTX)
- build_report.md now records packaging tool availability, fallback
  policy, offline guarantee, and verification entrypoints
- artifacts_manifest.json with sha256 checksums for packages + samples
- CHANGELOG entries for the welcome screen and renderer fixes

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

build/build_report.md

@@ -0,0 +1,43 @@
+# StepForge Build Report
+
+Version: 0.1.0
+Generated: 2026-06-11T02:33:30.201Z
+Host: linux x64 (node v20.20.2)
+
+## Outputs
+
+- Portable tarball: artifacts/stepforge_0.1.0_linux-x64.tar.gz
+- Debian package: artifacts/stepforge_0.1.0_amd64.deb
+- Sample guide archive: ../examples/sample-guide.sfgz
+- Sample exports (9 formats): see examples/sample-exports/
+- Full artifact list with sha256 checksums: artifacts_manifest.json
+
+## Packaging tool availability
+
+| Tool | Status |
+|---|---|
+| dpkg-deb (Linux .deb) | available |
+| rpmbuild (Linux .rpm) | **missing** |
+| appimagetool (Linux AppImage) | **missing** |
+| makensis (Windows installer .exe) | **missing** |
+| wixl / WiX (Windows .msi) | **missing** |
+
+Fallback policy: when a packaging tool is missing the build still produces
+the runnable app (portable tarball with launcher) plus whatever package
+formats the available tools allow. Windows artifacts are produced by
+`npm run package:windows` (electron-builder, portable .exe); .msi/.rpm/
+AppImage require the tools listed above and are skipped on this host.
+
+## Offline guarantee
+
+- The shipped app opens no sockets: no telemetry, update checks, license
+  checks, cloud sync, or remote AI. See SECURITY.md.
+- All exporters (PNG/GIF/PDF/DOCX/PPTX/ZIP) are implemented in-repo with
+  Node built-ins; Electron is the only third-party dependency
+  (dev-time fetch recorded in build/agent_audit.md).
+
+## Verification
+
+- `bash tests/run_test.sh` runs the workflow suites (node --test), a
+  startup smoke test of the Electron launcher, the sample-artifact
+  pipeline, and this release build.