Decide on darwin/amd64 (Intel Mac) release + upgrade coverage

[Lillevang]

Split out of #76 (self-upgrade retroactive spec) as a separate, independent item.

Context

The release matrix builds linux amd64/arm64, darwin arm64, and windows amd64 — but not darwin/amd64. The new agent-init upgrade command (PR #73) makes this gap user-visible for the first time: an Intel Mac running upgrade selects no matching asset and hits a "no asset for platform" error.

Decision needed

Pick one:

1. Add darwin/amd64 to the release matrix in .github/workflows/release.yml so Intel Macs get a published binary and upgrade works for them.
2. Don't support Intel Mac, but improve the upgrade error so it clearly says the platform isn't published (and points to building from source / go install), rather than a generic "no asset" message.

Notes

• This is a product-scope call, not a code bug. It existed before PR feat: add self-upgrade subcommand (agent-init upgrade) #73; that PR just exposed it.
• Whichever way it goes, it's a separate change from the self-upgrade merge.