Adopt repo standards: scaffold, policies, lint-clean (closes #1)

Add the standard scaffold and bring the tree to a clean lint under the
vendored `default: all` config: `script/` Scripts-to-Rule-Them-All
entrypoints with the `Makefile` as thin shims; a `Dockerfile` whose
`lint` and `test` phases gate the build; `.gitea/workflows/` CI running
`script/cibuild`; `REPO_POLICIES.md`, `.editorconfig`, `.dockerignore`,
`LICENSE` (WTFPL), `TODO.md`, `.gitignore`. The `.golangci.yml` is
byte-identical to the canonical copy in the `prompts` repo
(`https://git.eeqj.de/sneak/prompts/raw/branch/main/.golangci.yml`).

The 211 lint findings were fixed, not suppressed: package globals became
functions/fields/a command constructor, magic numbers became named
constants, `ctx` threads into the probes, loop functions were split to
cut complexity. Behavior is unchanged; the log file mode stays `0644`.
Four `//nolint:gosec` remain — G204 on the fixed-argv subprocess calls,
G304 on the operator-chosen log file — matching the reference repos.

`make check` is green (lint and tests run in Docker).

Model: opus-4-8

cmd/rtnetmon/main.go

@@ -1,20 +1,18 @@
 //go:build linux
-// +build linux
 
-// netmon – dual-interface network dashboard (curses)
-// WTFPL – 2025-05-16  sneak@sneak.berlin
+// Command rtnetmon is a dual-interface real-time network monitoring
+// dashboard for Linux. WTFPL, sneak@sneak.berlin.
 package main
 
 import (
 	"log"
-	"os"
 
 	"git.eeqj.de/sneak/rtnetmon/internal/cli"
 )
 
 func main() {
-	if err := cli.Execute(); err != nil {
+	err := cli.Execute()
+	if err != nil {
 		log.Fatal(err)
-		os.Exit(1)
 	}
 }