refactor: internalise playbook functions

[jacksyrm]

Summary

• Move inline playbook PowerShell blocks into named task scripts so YAML stays declarative and scripts are reusable/testable.
• Split large Atlas PowerShell modules into domain files while keeping the existing public module exports.
• Move wrapper implementation details into Scripts/Internal and leave existing public entrypoints as thin delegators.
• Harden new-user setup markers, taskbar pinning, and search taskbar mode finalization to avoid repeated setup after profile recreation.

Commit Structure

• refactor: move playbook inline scripts into tasks
• refactor: split Atlas modules by domain
• refactor: internalize script wrapper logic
• fix: harden new user setup completion

Verification

• Ran git diff --check new-update..refactor/internalise-functions.

[TheyCreeper]

lgtm