fix(hotkey): split platform files by platform+cgo build matrix

The `!darwin` guard on the modifier helpers was too broad: golang.design/x/
hotkey only exposes its Mod*/Key* constants from the darwin&&cgo, linux&&cgo
and windows backends, while the `!windows && !cgo` complement (e.g. Linux
cross-builds with CGO_ENABLED=0) falls back to a stub with no constants. That
made hk.ModCtrl/ModShift/KeyA... undefined and produced editor/vet warnings.

Realign the build tags with the upstream matrix and confine every hk.* constant
to a tagged file so the shared hotkey.go compiles everywhere. Also fix the
Linux mapping (option -> hk.Mod1; hk.ModAlt does not exist on X11).

internal/infrastructure/hotkey/hotkey_linux.go

@@ -0,0 +1,18 @@
+//go:build linux && cgo
+
+package hotkey
+
+import hk "golang.design/x/hotkey"
+
+// On Linux we map "cmd" to Ctrl so the same config string ("cmd+shift+a")
+// stays meaningful across platforms.
+//
+// The X11 backend exposes modifiers as Mod1..Mod5 rather than named Alt/Option
+// constants; Mod1 is the conventional Alt mask under X11, so "option"/"alt"
+// resolve to hk.Mod1. This file is gated on cgo because the constants only
+// exist in the cgo-backed linux implementation (CGO_ENABLED=0 falls back to a
+// stub with no constants).
+func modCmd() hk.Modifier    { return hk.ModCtrl }
+func modCtrl() hk.Modifier   { return hk.ModCtrl }
+func modOption() hk.Modifier { return hk.Mod1 }
+func modShift() hk.Modifier  { return hk.ModShift }