Epic: Make common user entry points install LKG

## Goal

After `lkg` is promoted to `v0.0.70`, make every common user entry point install that maintained release.

The hosted installer already defaults to `lkg`, and the repo plus live NVIDIA quickstarts already use that installer. This Epic owns the remaining release-day corrections and verification, not a new release-management platform.

## Child work

- #5959 — promote `v0.0.70` to `lkg` and verify clean public install plus update.
- #6063 — make the documented public Brev launchable follow `lkg` automatically.
- #6064 — remove stale documentation pins and verify repo docs, NVIDIA docs, the DGX Spark playbook, and the AI-agent skill route through the hosted LKG installer.

## Definition of done

- [ ] `refs/tags/lkg` and `v0.0.70^{commit}` resolve to `8120223922bf6a501df32a8e269ce9dcf2180819`.
- [ ] A clean `https://www.nvidia.com/nemoclaw.sh` install with no version override checks out that commit under `~/.nemoclaw/source`.
- [ ] `nemoclaw update --yes` from an older maintained install checks out that commit.
- [ ] Repo docs, live NVIDIA docs, and the DGX Spark playbook use the hosted installer without a stale concrete release pin.
- [ ] The compact user-guide skill routes installation requests to the live quickstarts.
- [ ] The documented Brev launchable deploys the current LKG and its production image rebuilds automatically when LKG changes.

## Release-day order

1. Prepare and validate the docs and Brev changes.
2. Stage the Brev image from `v0.0.70`.
3. Move `lkg` and verify hosted install plus update.
4. Dispatch the Brev LKG production build and smoke-test the documented launchable.
5. Verify the published docs, DGX Spark playbook, and AI-agent route.

## Non-goals

- A protected LKG promotion control plane.
- A release-consumer registry or drift-monitoring service.
- A new `/lkg` documentation channel.
- A general provenance, image-contract, or rollback framework.
- Rewriting historical release notes.