feat: publish Linux and macOS CLI installer

[ashishmax31]

Summary

• make the shell installer agent-friendly with explicit version and install-directory flags, checksum verification, and atomic replacement
• select a safe writable install directory already on PATH when possible
• persist $HOME/.local/bin for zsh, Bash, and POSIX shells, support GitHub Actions through $GITHUB_PATH, and provide --no-modify-path
• narrow alpha release artifacts to Linux and macOS on AMD64 and ARM64
• publish versioned and stable installer scripts to the production R2 bucket behind get.stackdome.com
• make versioned installer objects immutable across reruns and enforce the exact GitHub release asset contract
• remove the Windows installer, workflow, tests, and repository AGENTS.md

Infrastructure

• Cloudflare R2 bucket: stackdome-installers-prod
• custom domain: https://get.stackdome.com
• GitHub environment: installer-production
• required repository secrets and variables are configured

The stable installer URL remains empty until the first release tag containing this workflow is published. Tag the merged commit, for example v0.0.2-alpha, to build release assets and publish cli.sh.

Verification

• go test ./... -count=1
• go test -race ./... -count=1
• go vet ./...
• go build ./cmd/stackdome
• sh -n install.sh
• shellcheck install.sh
• actionlint
• git diff --check origin/main...HEAD
• manual PATH persistence, idempotence, no-modify, GITHUB_PATH, unset-shell, and macOS Bash profile precedence smokes