Adds a proper feature page at user-guide/features/goals.md covering
the /goal slash command — Hermes' take on the Ralph loop shipped in
PR #18262. The slash-commands reference table had two table rows but
no narrative doc walking through the judge model, fail-open semantics,
turn budget, persistence, user-message preemption, or the aux-model
config override.
Adds a walkthrough example showing a multi-turn goal running to
completion, covers the two judge failure modes with how to recover,
and credits Codex CLI 0.128.0 / Eric Traut as prior art.
Also cross-links both slash-commands.md rows to the new page so
readers discovering /goal from the command reference can dive in.
