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.

modules/nft_payload_cve_2023_0179/NOTICE.md

@@ -20,7 +20,7 @@ Upstream fix: mainline 6.2-rc4 (commit `696e1a48b1a1`, Jan 2023).
 Branch backports: 4.14.302 / 4.19.269 / 5.4.229 / 5.10.163 /
 5.15.88 / 6.1.6.
 
-## IAMROOT role
+## SKELETONKEY role
 
 userns+netns. Hand-rolled nfnetlink batch: NEWTABLE → NEWCHAIN →
 NEWSET with `NFTA_SET_DESC` describing variable-length elements →