Install guides for Nix, Ubuntu, and Windows + release workflow for static binaries

[sdelcore]

Why

The README only had a NixOS dev-shell quickstart — no way for someone on Ubuntu, Windows, or any non-Nix machine to install the platform, and the flake's package + NixOS module were undocumented despite already working.

What

• README ## Install section, split by role — hosting the server vs using the CLI:
  • Hosting (Linux only)
    • Nix: nix profile install github:sdelcore/shared, plus a copy-paste services.shared NixOS module example (the module already existed in the flake, just wasn't documented).
    • Ubuntu / other Linux: release tarball or CGO_ENABLED=0 go install (with a note that Go 1.24+ is newer than Ubuntu 24.04's apt package), plus a systemd unit mirroring the NixOS module (DynamicUser, StateDirectory, /var/lib/shared).
  • CLI (any machine)
    • Linux/macOS: the release tarball or go install .../cmd/shared@latest.
    • Windows: CLI only — the server intentionally does not ship for Windows. Release zip contains just shared.exe; point it at the server with SHARED_SERVER.
  • Also documents the homepage's × delete button alongside the sites list.
• skills/install-shared-cli/: an agent skill (Claude Code or any SKILL.md-compatible agent) that installs the CLI on whatever OS it's running on — Nix → Go → release binary, then sets SHARED_SERVER and smoke-tests with shared list. README shows the one-liner to copy it into ~/.claude/skills/.
• .github/workflows/release.yml: on v* tags, cross-compiles static binaries (CGO_ENABLED=0 -trimpath -ldflags "-s -w"): linux amd64/arm64 tarballs with sharedd + shared, windows amd64/arm64 zips with shared.exe only. Publishes with gh release create --generate-notes.
• shared open now picks the opener per OS (rundll32 on Windows, open on macOS, xdg-open elsewhere) instead of hardcoding xdg-open — the only Windows-incompatible code in the repo.

Verified

• go build, go vet, go test ./... pass; cross-compiles clean for linux, windows, and darwin (pure Go, no cgo).
• shared init scaffolds correctly and skips existing files on re-run.
• Ran the workflow's exact build/package loops locally: linux tarballs contain statically linked, stripped binaries (file confirms — no glibc coupling, unlike the nix-built binary); the windows zip contains a single valid PE32+ shared.exe.
• Workflow passes actionlint.
• .github/workflows/ci.yml: on pushes to main and PRs — go vet, go test, cross-compile check for linux/windows/darwin, and nix build (catches stale vendorHash when deps change). Both jobs green on this PR.