feat: add cross-platform installers and actionable build diagnostics

[ashishmax31]

Summary

• add checksum-verified Stackdome CLI installers for macOS, Linux, and Windows, with installation docs and native test coverage
• make release publishing manually repairable and keep new releases in draft until archives and checksums are fully uploaded
• add pull-request CI for CLI, Unix installer, and release-contract coverage, plus Windows PowerShell 5.1/7 installer CI
• show copyable build references in build list and both the reference and full ID in build info
• accept displayed build references in build info and build logs
• preserve streaming API error reasons internally and render concise messages for pending or pruned build logs

This supersedes the CLI distribution work in #3; PR #3 remains obsolete and should not be merged.

Verification

• go test ./... -count=1
• go test -race ./... -count=1
• go vet ./...
• actionlint .github/workflows/*.yml
• sh -n install.sh
• go build ./cmd/stackdome