Add CLI support for custom OpenAI-compatible endpoints and remove TUI (#2710)

* feat(model): add `picoclaw model add` for custom OpenAI-compatible endpoints

Onboards a model from a user-supplied API base + key by hitting
GET <base>/models, prompting the user to pick one, and writing the entry
into model_list[] (with api_keys) plus setting it as the default model.
This was previously only available in the TUI launcher (issue #2208) and
is now accessible from the CLI:

    picoclaw model add -b URL -k KEY [-m MODEL] [-n ALIAS]

* chore: remove deprecated picoclaw-launcher-tui

Per RFC #2208, the TUI launcher is deprecated in favor of the CLI; its
"online model picker" feature has been ported to `picoclaw model add` in
the previous commit. This drops the binary and all build/release/docs
references:

- delete cmd/picoclaw-launcher-tui/ and assets/launcher-tui.jpg
- Makefile: remove the `build-launcher-tui` target
- .goreleaser.yaml: drop the build entry plus the `picoclaw-launcher-tui`
  ids from the launcher docker image, macOS notarize list, and nfpms
  contents
- docker/Dockerfile.goreleaser.launcher: drop the COPY for the TUI binary
- READMEs (root + 8 locales): remove the "TUI Launcher" section and
  screenshot link
- docs/guides/docker.*: update the "launcher image includes …" sentence
  to reflect the two remaining binaries

`make build` still succeeds; `go build ./web/backend` (the launcher
target) still succeeds. `picoclaw-launcher` (web console) is unaffected.

docker/Dockerfile.goreleaser.launcher

@@ -6,7 +6,6 @@ RUN apk add --no-cache ca-certificates tzdata
 
 COPY $TARGETPLATFORM/picoclaw /usr/local/bin/picoclaw
 COPY $TARGETPLATFORM/picoclaw-launcher /usr/local/bin/picoclaw-launcher
-COPY $TARGETPLATFORM/picoclaw-launcher-tui /usr/local/bin/picoclaw-launcher-tui
 
 ENTRYPOINT ["picoclaw-launcher"]
 CMD ["-console", "-public", "-no-browser"]