Docs: arm64 / non-amd64 runners and AGENTCTL_INSTALL=release tarball

[leo-aa88]

Context

The release install path in .github/workflows/agentctl-pr-review.yml (and agentctl-pr-review-publish.yml) downloads:

agentctl-<tag>-linux-amd64.tar.gz

That matches GitHub-hosted ubuntu-latest (x86_64). Teams using self-hosted ARM64 (or other) runners will not get a usable binary from that asset; the install step fails loudly if /tmp/agentctl is missing after extract (good), but docs do not say why or what to do.

Proposal

Add a short subsection under Installing agentctl in Actions in docs/GITHUB_ACTIONS.md:

• Release assets are linux-amd64 only today.
• On arm64 (or other) self-hosted runners, use AGENTCTL_INSTALL: go-build on a checkout that includes the Go module, or supply a custom install step / your own asset URL.

Acceptance criteria

• Someone on arm64 self-hosted finds the limitation and the workaround in one place.