retire gha release workflow

[mokagio]

Rationale

Stacked on top of #71.
With the Buildkite pipeline in #71 doing the full build+sign+notarize+publish path on every v* tag, leaving the GHA release workflow live would race the BK publish and attach two assets to the same release — one signed and notarized (BK), one unsigned and arm64-only (GHA).

Asset names line up after the swap, so downstream consumers see the same shape they'd see today, just with -universal instead of -arm64:

	GHA (today)	BK (after #71 lands)
Tarball	imessage-cli-${version}-macos-arm64.tar.gz	imessage-cli-${version}-macos-universal.tar.gz
Companion	…tar.gz.sha256	…tar.gz.sha256
Tar contents	bare imessage-cli at root	bare imessage-cli at root

Tradeoffs

• .github/workflows/ci.yml is intentionally untouched. It runs tests and npm publish to GitHub Packages on every main push — that's a separate concern from the CLI release and isn't being replaced by Buildkite.

Gotchas

• This PR assumes add buildkite pipeline for cli release #71's first tag-trigger BK build has been verified end to end. Build #7 already proved the build+sign+notarize path on a non-tag commit (notarization id 77445786-…, CDHashes match Apple's ticket). The publish step (gh release create + gh release upload --clobber) will only fire on the first v* tag — would be worth eyeballing the first one, since gh auth on the Mac agent is the one piece we haven't exercised yet.

How to test

Land #71 first.
After this PR merges, push a test tag (e.g. bump package.json and tag vX.Y.Z) and confirm:

• Only the BK pipeline fires (no GHA release workflow run on the Actions tab).
• The release published to GitHub has exactly the universal asset + sha256 — no leftover -arm64 asset alongside.

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Posted by Claude Code (Opus 4.7, 1M context) on behalf of @mokagio with approval.

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