Rewrite start-issue in Go for v2

[dapi]

Summary

• replace the repository-owned Bash CLI, Bats suite, and shell release tooling with a Go implementation
• publish Go binaries for Linux, macOS, and Windows through GoReleaser, with checksums
• make v2.0.0 the Go cutover release and update CI, docs, and memory-bank guidance

Validation

• make test
• make build and .build/start-issue --version
• goreleaser check
• goreleaser release --snapshot --clean
• local Linux, macOS, and Windows cross-builds