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/MODULE.md

@@ -25,7 +25,7 @@ by them.
 
 Even in 2026, many production deployments still run vulnerable
 kernels (RHEL 7/8, older Ubuntu LTS, embedded). Bundling Dirty Pipe
-makes IAMROOT useful as a "historical sweep" tool on long-tail
+makes SKELETONKEY useful as a "historical sweep" tool on long-tail
 systems.
 
 ## Implementation plan
@@ -34,8 +34,8 @@ systems.
   `NOTICE.md` when implemented)
 - `detect()`: kernel version check + `/proc/version` parse + test
   for fixed-version backports
-- `exploit()`: writes `iamroot::0:0:dirtypipe:/:/bin/bash` into
-  `/etc/passwd`, then `su iamroot` — same shape as copy_fail's
+- `exploit()`: writes `skeletonkey::0:0:dirtypipe:/:/bin/bash` into
+  `/etc/passwd`, then `su skeletonkey` — same shape as copy_fail's
   backdoor mode
 - Detection rules: auditd on splice() calls + pipe write patterns,
   filesystem audit on `/etc/passwd` modification by non-root
@@ -44,4 +44,4 @@ systems.
 
 Pick this up after Phase 1 (module-interface refactor of the
 copy_fail family) so this module can use the standard
-`iamroot_module` shape from the start.
+`skeletonkey_module` shape from the start.