feat: configure CLI installer PATH

[ashishmax31]

Summary

• prefer a safe writable install directory that is already on PATH
• persist the fallback $HOME/.local/bin path for zsh, Bash, and POSIX shells
• preserve macOS Bash startup-file precedence
• update $GITHUB_PATH for GitHub Actions
• add --no-modify-path for agents and CI that manage PATH themselves
• print a safely quoted export command for the current shell

No new tests were added. Existing installer tests and manual profile/path smokes cover the launch verification.

Verification

• go test ./... -count=1
• go test -race ./... -count=1
• go vet ./...
• go build ./cmd/stackdome
• sh -n install.sh
• shellcheck install.sh
• actionlint
• manual PATH persistence, idempotence, no-modify, GITHUB_PATH, empty-shell, safe quoting, and macOS Bash profile precedence smokes