feat(packaging): ship punktfunk everywhere, and deprecate the session's stray pairing flag

The rest of C1. A CLI that only exists in a dev tree is not a door anyone can use, so it
now builds and installs in deb, rpm, arch, nix, flatpak and MSIX alongside the client and
the session. On Windows it gets its own `AppListEntry="none"` application with the alias
`punktfunk.exe` — a command, not a Start-menu tile — which is what the Playnite importer
will shell to for `punktfunk library <host> --json`.

`punktfunk-session --pair` prints a deprecation notice and forwards. Pairing is a trust
ceremony and belongs to the brain, fronted by `punktfunk pair` or a shell; a renderer
owning one is precisely the mixing of concerns this split exists to undo. It keeps working
for a release, because someone's provisioning script is calling it today.

Decky's invocations are untouched and verified still working (`--list-hosts` JSON shape
unchanged, `--reachable` exit codes unchanged): those flags are a frozen compat contract
until Decky migrates to the verbs at its own pace.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

packaging/flatpak/io.unom.Punktfunk.yml

@@ -222,8 +222,12 @@ modules:
       # unification identical to a workspace build (the pinned skia-binaries archive's
       # `pdf-textlayout-vulkan` key must match the resolved feature set).
       - cargo --offline build --release -p punktfunk-client-linux -p punktfunk-client-session
+          -p punktfunk-cli
       - install -Dm0755 target/release/punktfunk-client ${FLATPAK_DEST}/bin/punktfunk-client
       - install -Dm0755 target/release/punktfunk-session ${FLATPAK_DEST}/bin/punktfunk-session
+      # The headless CLI (design/client-architecture-split.md §4) — reachable as
+      # `flatpak run --command=punktfunk io.unom.Punktfunk …`.
+      - install -Dm0755 target/release/punktfunk ${FLATPAK_DEST}/bin/punktfunk
       # Desktop entry (renamed to the app id; Exec is the in-sandbox binary).
       - install -Dm0644 packaging/flatpak/io.unom.Punktfunk.desktop
           ${FLATPAK_DEST}/share/applications/io.unom.Punktfunk.desktop