rename: IAMROOT → SKELETONKEY across the entire project

Breaking change. Tool name, binary name, function/type names,
constant names, env vars, header guards, file paths, and GitHub
repo URL all rebrand IAMROOT → SKELETONKEY.

Changes:
  - All "IAMROOT" → "SKELETONKEY" (constants, env vars, enum
    values, docs, comments)
  - All "iamroot" → "skeletonkey" (functions, types, paths, CLI)
  - iamroot.c → skeletonkey.c
  - modules/*/iamroot_modules.{c,h} → modules/*/skeletonkey_modules.{c,h}
  - tools/iamroot-fleet-scan.sh → tools/skeletonkey-fleet-scan.sh
  - Binary "iamroot" → "skeletonkey"
  - GitHub URL KaraZajac/IAMROOT → KaraZajac/SKELETONKEY
  - .gitignore now expects build output named "skeletonkey"
  - /tmp/iamroot-* tmpfiles → /tmp/skeletonkey-*
  - Env vars IAMROOT_MODPROBE_PATH etc. → SKELETONKEY_*

New ASCII skeleton-key banner (horizontal key icon + ANSI Shadow
SKELETONKEY block letters) replaces the IAMROOT banner in
skeletonkey.c and README.md.

VERSION: 0.3.1 → 0.4.0 (breaking).

Build clean on Debian 6.12.86. `skeletonkey --version` → 0.4.0.
All 24 modules still register; no functional code changes — pure
rename + banner refresh.

core/kernel_range.c

@@ -1,5 +1,5 @@
 /*
- * IAMROOT — kernel_range implementation
+ * SKELETONKEY — kernel_range implementation
  */
 
 #include "kernel_range.h"
@@ -19,7 +19,7 @@ bool kernel_version_current(struct kernel_version *out)
     if (uname(&u) < 0) return false;
 
     /* Stash release string for callers that want to print it. We hold
-     * a single static buffer; not threadsafe but iamroot is single-
+     * a single static buffer; not threadsafe but skeletonkey is single-
      * threaded today. */
     snprintf(g_release_buf, sizeof(g_release_buf), "%s", u.release);
     out->release = g_release_buf;