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.

Generated with [Devin](https://devin.ai)

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

CONTRIBUTING.md

@@ -0,0 +1,58 @@
+# Contributing
+
+Thanks for considering a contribution. Dash is a small, deliberate codebase —
+read [AGENTS.md](AGENTS.md) and [ROADMAP.md](ROADMAP.md) first; the roadmap is
+canonical for what gets built and when.
+
+## Setup
+
+Requires Go 1.24+ and Node 20+.
+
+```sh
+npm install
+npm run generate       # regenerate the TS client from openapi.yaml
+npm run dev            # vite dev server on :5173, proxies /api to :8080
+```
+
+```sh
+cd apps/backend
+DASH_DEV=1 go run ./cmd/dash   # API on :8080
+```
+
+## Rules of the house
+
+- `openapi.yaml` is the API contract. Change it first, then
+  `npm run generate`. Never hand-write API types in the frontend.
+- All schema changes go through goose migrations in
+  `apps/backend/internal/db/migrations` — `NNNN_name.sql` with `-- +goose Up`
+  and `-- +goose Down`.
+- Board renderers share state, drag-drop, and popover primitives. Never
+  reimplement them per renderer.
+- No auth, no YAML-as-source-of-truth, no external database. These are
+  non-goals for v1.
+- Standard library before dependencies; existing helpers before new ones.
+
+## Before pushing
+
+```sh
+npm run typecheck
+npm run lint
+npm run build
+
+cd apps/backend
+go build ./... && go vet ./... && go test ./...
+```
+
+Plus a real smoke test of whatever you changed. All gates green or no push.
+
+## Commits and PRs
+
+- Conventional-ish subjects: `feat:`, `fix:`, `docs:`, `refactor:` — keep them
+  short and describe the why.
+- Small, reviewable PRs. Fill in the template; a screenshot of UI changes is
+  worth more than a paragraph.
+
+## Reporting bugs / requesting features
+
+Use the issue templates. For security issues, see
+[SECURITY.md](SECURITY.md) — do not open a public issue.