docs: add README.sethlabels.md (fork entry point)

README.sethlabels.md

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+# sethLabels
+
+> Deployment fork of [glabels-qt](https://github.com/j-evins/glabels-qt) — Qt6
+> label designer / printer, packaged for Debian-family Linux and macOS.
+
+This is **not** a code fork. The upstream application is unchanged; sethLabels
+exists solely to publish installable binary artifacts that upstream explicitly
+does not provide ("Currently there are no self-hosted binary snapshot releases
+available… I encourage you to try building the code yourself" — upstream README).
+
+For the application itself — what it does, screenshots, full feature list — see
+the upstream [`README.md`](README.md).
+
+## Install
+
+### Debian / Ubuntu (`.deb`)
+
+Download the latest `.deb` from the [releases page](https://git.sethpc.xyz/Seth/sethLabels/releases),
+then:
+
+```
+sudo apt install ./glabels-qt_<VERSION>_amd64.deb
+glabels-qt --version
+```
+
+### Any Linux (AppImage)
+
+Download `sethlabels-gui-<VERSION>-x86_64.AppImage` from the [releases page](https://git.sethpc.xyz/Seth/sethLabels/releases),
+make it executable, and run it:
+
+```
+chmod +x sethlabels-gui-<VERSION>-x86_64.AppImage
+./sethlabels-gui-<VERSION>-x86_64.AppImage
+```
+
+A separate `sethlabels-batch-<VERSION>-x86_64.AppImage` provides the CLI for
+scripted / mail-merge use.
+
+### macOS (Homebrew)
+
+```
+brew tap seth/tap https://git.sethpc.xyz/Seth/homebrew-tap.git
+brew install seth/tap/glabels-qt
+```
+
+The explicit URL form is needed because brew defaults to GitHub for tap names.
+First install builds Qt6 + glabels-qt from source (~5–10 min one-time cost; see
+spec §D2). Subsequent updates are a fast `brew upgrade`.
+
+## Build from source
+
+If you'd rather build the artifacts yourself instead of downloading a release:
+
+```
+git clone https://git.sethpc.xyz/Seth/sethLabels.git
+cd sethLabels
+./scripts/lib/deps-debian.sh        # check / install build deps
+./scripts/build-deb.sh              # → build/deb/glabels-qt_*.deb
+./scripts/build-appimages.sh        # → sethlabels-{gui,batch}-*.AppImage
+```
+
+See [`scripts/README.md`](scripts/README.md) for full operator docs.
+
+## How this fork works
+
+sethLabels is a **deployment fork**: every sethLabels addition lives in NEW
+files in NEW top-level directories (`scripts/`, `packaging/`,
+`sethlabels-docs/`, `tests-impl/`, plus this file). Upstream files are never
+edited. The single allowlisted exception is `.gitignore`. This discipline is
+enforced by `scripts/check-no-upstream-edits.sh`.
+
+The `<upstream-tag>-seth<N>` versioning preserves the upstream-lineage in every
+artifact. Periodic `git rebase upstream/master` is conflict-free by construction.
+
+## Spec & decisions
+
+- [Design spec](sethlabels-docs/specs/2026-04-29-packaging-design.md) — invariants, decisions, build pipeline, failure modes
+- [Decision log](DECISIONS.md) — settled choices + rejected alternatives
+- [Project brief](IDEA.md) — plain-language motivation
+
+## License
+
+The upstream code is licensed under [GPL-3.0](LICENSE). sethLabels-specific
+files (everything in the dirs listed above, plus this file) are licensed under
+the same terms.
+
+## Upstream
+
+- Upstream: https://github.com/j-evins/glabels-qt (Jaye Evins / glabels.org)
+- This fork: https://git.sethpc.xyz/Seth/sethLabels
+- Brew tap: https://git.sethpc.xyz/Seth/homebrew-tap