update-backend-configurationlisted

# Update the Backend Configuration Switching backends is a write, and a write deserves a preflight: a stack persisted for a CLI that is not on the machine turns every later mission into the same discovery. So this skill checks the binary first, writes second, and proves nothing itself — the proof is `check-backend-configuration`'s job, and step 5 hands it over. Three lines this skill never crosses: it never installs anything silently (it offers, the user runs or approves), it never authenticates on the user's behalf, and it never stores a secret — credentials stay in the CLI's own auth store and are referred to by name only. This skill knows no stack by name. The valid stacks, their aliases, their CLIs, and what each persists come from the `observability-cli-guides` skill: `references/builtin-stacks.md` for the list, `references/<stack>.md` for the stack — its `## CLI binary` section for the preflight, its `## What to persist` section for the `stack_config` write. ## 1. Resolve the target stack The target is one of the `STACKS` values `builtin-stacks.md` lists — `odd_config_set` accepts nothing else. Map the user's phrasing onto a row through that table's **Also called** column — a vendor name, a product name, or the words for the stack on this machine each name exactly one value there. Anything that does not resolve is an **error naming the valid list** (the table's first column), not a guess and not a passthrough: `odd_config_set` would reject it anyway, and a spelled