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/dirty_pipe_cve_2022_0847/detect/sigma.yml

@@ -1,5 +1,5 @@
 title: Possible Dirty Pipe exploitation (CVE-2022-0847)
-id: f6b13c08-iamroot-dirty-pipe
+id: f6b13c08-skeletonkey-dirty-pipe
 status: experimental
 description: |
   Detects file modifications to /etc/passwd, /etc/shadow, /etc/sudoers,
@@ -10,7 +10,7 @@ description: |
 references:
   - https://dirtypipe.cm4all.com/
   - https://nvd.nist.gov/vuln/detail/CVE-2022-0847
-author: IAMROOT
+author: SKELETONKEY
 date: 2026/05/16
 logsource:
   product: linux