Pin ecosystem crates and switch to bread-utils::singleton

Repoint bread-theme and bread-utils at git.breadway.dev tag v0.7.1
(no github.com). bread-screenshots is not on that tag yet, so rev-pin
it instead of tracking branch=main.

Replace the homegrown PID-file toggle with bread-utils::singleton so
the launcher still dismisses on a second keybind without the TOCTOU
window. Screenshot mode still skips the lock.

README now documents the fixed BOS dark fallback (not Catppuccin
Mocha). Drop the unused GitHub Actions release workflow; Forgejo is
canonical. CONTRIBUTING already documents single-trunk.

README.md

@@ -14,7 +14,7 @@ breadbox          GTK4 layer-shell launcher
 - Reads the active Hyprland workspace and sorts apps by context priority
 - Fuzzy filtering as you type; Enter launches, Escape closes
 - App icons loaded from the resolved icon cache (see `breadbox-sync`)
-- pywal palette auto-detected from `~/.cache/wal/colors.json`, falls back to Catppuccin Mocha
+- pywal accents from `~/.cache/wal/colors.json`; background/surface/overlay/foreground stay fixed BOS dark
 - User CSS override at `~/.config/breadbox/style.css`
 - Toggle/dismiss: running a second instance kills the first