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/MODULE.md

@@ -25,10 +25,10 @@ polkit until 0.121 (or distro backport).
   - Debian: 0.105-31+deb11u1 (bullseye), 0.105-26+deb10u1 (buster)
   - RHEL: polkit-0.115-13.el7_9 (RHEL 7), polkit-0.117-9.el8_5.1 (RHEL 8)
 
-## IAMROOT detect logic (current)
+## SKELETONKEY detect logic (current)
 
 1. Resolve pkexec binary (`/usr/bin/pkexec` or `which pkexec`)
-2. If not present → IAMROOT_OK (no attack surface)
+2. If not present → SKELETONKEY_OK (no attack surface)
 3. Run `pkexec --version` and parse version
 4. Compare to known-fixed thresholds; report VULNERABLE if below