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/pwnkit_cve_2021_4034/NOTICE.md

@@ -14,7 +14,7 @@ Original advisory:
 
 Upstream fix: polkit 0.121 (Jan 2022).
 
-## IAMROOT role
+## SKELETONKEY role
 
 The exploit module follows the canonical Qualys-style chain: writes
 payload.c + gconv-modules cache, compiles via the target's gcc,
@@ -22,4 +22,4 @@ execve's pkexec with NULL argv and crafted envp. Handles both the
 legacy ("0.105") and modern ("126") polkit version string formats.
 Falls back gracefully on hosts without a compiler.
 
-This is IAMROOT's first **userspace** LPE — not a kernel bug.
+This is SKELETONKEY's first **userspace** LPE — not a kernel bug.