Add manual binary build workflow

[mintyPT]

Summary:

• add a separate GitHub Actions workflow for building installable Linux binaries
• run on pull requests, pushes to main, and manual dispatch
• upload amd64, arm64, and checksums.txt as a workflow artifact instead of publishing a GitHub Release

Context:
This is intentionally separate from the Kamal hook support PR. It gives branches a way to produce downloadable binaries before an official release tag is cut.

Tests:

• env GOCACHE=/tmp/singleserver-go-build go test ./...
• local cross-compile smoke test for linux/amd64 and linux/arm64 using the same ldflags pattern as the workflow