Keep screensaver/lock from engaging during Steam games

[Rooke]

Summary

• Steam's client window already inhibits idle while fullscreen (default/hypr/apps/steam.lua), but the actual game runs in a separate window that Steam classes steam_app_<appid> (true for both native Linux titles and Proton). That window wasn't covered, so the screensaver and lock still engaged mid-game — controller/gamepad input doesn't register as user activity to the compositor.
• Adds one window rule following the exact pattern already used for Steam's own window and for RetroArch/Moonlight/GeForce Now: idle_inhibit = "fullscreen" scoped to ^steam_app_[0-9]+$, so idle suppression is tied to the game window actually being on screen, not to input.

Test plan

• Added test/shell.d/steam-idle-inhibit-test.sh asserting the new rule is present
• ./test/all — no new failures (5 pre-existing, unrelated failures reproduce identically on a clean checkout of this branch's base: missing sibling omarchy-pkgs checkout, two scripts losing their exec bit through the fork/clone, and one shell-IPC sandbox flake)
• Verified manually against Hyprland's own window-rule semantics; idleinhibit fullscreen, class:^(steam_app_) is the documented community-standard fix for this exact issue (see https://forum.hypr.land/t/window-rule-for-games/988)

🤖 Generated with Claude Code

https://claude.ai/code/session_015Pt2Qi6SP9TN6Rmr9LiL4V

[bjarneo]

Automated duplication check: this pull request looks similar to #9667, which covers the same fullscreen idle-inhibit for Steam game windows. I keep that one open and close this one to consolidate review.

If you feel this is the wrong decision, please open the PR again with a note on the difference.