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

TODO.md

@@ -31,6 +31,11 @@ P2: security: per-IP rate limiting on the image routes
 
 # Completed Steps
 
+- 2026-10-04 `cmd/pixad/main.go` is one call into `internal/` (closes #206):
+  what it did (the command line and its `--config` flag, setting
+  `PIXA_CONFIG_PATH`, ignoring `SIGPIPE`, starting the fx app) is now `Run` in
+  `internal/app`, unchanged, and `main` calls it with `Version`, which the build
+  still sets through `-X main.Version`. That code had no tests to move.
 - 2026-10-04 `.gitignore` ignores `.claude/` (closes #204): the entry and its
   comment are copied from the canonical `.gitignore` in `sneak/prompts`,
   unanchored so it matches at every depth. `.dockerignore` already has