Move pixad's startup from cmd/pixad into internal/app (closes #206)

cmd/pixad/main.go built the command line and its --config flag, set
PIXA_CONFIG_PATH from that flag, ignored SIGPIPE and started the fx
app. REPO_POLICIES.md now requires cmd/ to hold only one call into
internal/ or pkg/, so that code moves unchanged to Run in the new
internal/app package, and main calls app.Run(Version). Version stays
in main, so the -X main.Version build flags in the Dockerfile and the
Makefile do not change.

Model: opus-5-5

internal/globals/globals.go

@@ -7,8 +7,8 @@ import (
 
 const appname = "pixad"
 
-// Version is populated from main() via ldflags.
-var Version string //nolint:gochecknoglobals // set from main
+// Version is set by app.Run to the version main was built with.
+var Version string //nolint:gochecknoglobals // set by app.Run
 
 // Globals holds application-wide constants.
 type Globals struct {