Add gs script for gentle-shell + per-PR worktrees

[barbatdev]

Problem

There is no way to run gs (the gentle-shell launcher and disposable-worktree helper for gentle-ai, gentle-shell and engram) from this dotfiles checkout. The gs fish alias for git status was already removed in a prior PR specifically to free the name for this tool, but the tool itself was never added.

Proposal

• Add the gs script verbatim at bin/gs.
• Document the manual one-time symlink step to ~/.local/bin/gs in the README, since install.sh does not currently manage a bin/ directory of standalone scripts.
• No new installer automation; this is intentionally a minimal, documented manual step.

Out of scope

• Building an automated installer/symlink mechanism for bin/.
• Changes to gs itself.

[barbatdev]

Delivered in #64.