docs+release: Phase 4 launch artifacts

README hero with real screenshots, install/config/widget docs, goreleaser
for dash + dash-agent, GHCR release workflow, changelog, community files,
agent systemd unit and Dockerfile, version stamping via -X main.version.

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Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

.github/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,11 @@
+## Summary
+
+<!-- What does this change and why? -->
+
+## Test plan
+
+- [ ] `npm run typecheck` + `npm run lint` + `npm run build` green
+- [ ] `go build ./...` + `go vet ./...` + `go test ./...` green
+- [ ] Smoke-tested the affected flow manually
+
+<!-- Screenshots for UI changes. -->