docs: fix script name in directory tree examples (#2555)

* docs: fix script name in directory tree examples

Replace update-claude-md.sh with the actual filename setup-tasks.sh
in two directory tree examples (Steps 2 and 4 of the detailed walkthrough).

* docs: fix .specify/scripts layout to show bash/ and powershell/ subdirs

install_shared_infra() installs scripts under .specify/scripts/bash/
(script_type="sh") or .specify/scripts/powershell/ (script_type="ps"),
not directly under .specify/scripts/. Update docs/installation.md and
the _install_shared_infra docstring to reflect the actual on-disk layout.

* docs: update README tree examples to show scripts/bash/ subdirectory

Scripts are installed under .specify/scripts/bash/ (or powershell/)
not directly under .specify/scripts/. Fix both tree diagrams in the
Detailed Process walkthrough to match the actual on-disk layout.