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.

core/registry.c

@@ -1,5 +1,5 @@
 /*
- * IAMROOT — module registry implementation
+ * SKELETONKEY — module registry implementation
  *
  * Simple flat array. Resized in chunks of 16. We never expect more
  * than a few dozen modules, so this is fine.
@@ -14,22 +14,22 @@
 
 #define REGISTRY_CHUNK 16
 
-static const struct iamroot_module **g_modules = NULL;
+static const struct skeletonkey_module **g_modules = NULL;
 static size_t g_count = 0;
 static size_t g_cap = 0;
 
-void iamroot_register(const struct iamroot_module *m)
+void skeletonkey_register(const struct skeletonkey_module *m)
 {
     if (m == NULL || m->name == NULL) {
-        fprintf(stderr, "[!] iamroot_register: NULL module or unnamed module\n");
+        fprintf(stderr, "[!] skeletonkey_register: NULL module or unnamed module\n");
         return;
     }
     if (g_count == g_cap) {
         size_t new_cap = g_cap + REGISTRY_CHUNK;
-        const struct iamroot_module **n =
+        const struct skeletonkey_module **n =
             realloc((void *)g_modules, new_cap * sizeof(*g_modules));
         if (n == NULL) {
-            fprintf(stderr, "[!] iamroot_register: OOM\n");
+            fprintf(stderr, "[!] skeletonkey_register: OOM\n");
             return;
         }
         g_modules = n;
@@ -38,18 +38,18 @@ void iamroot_register(const struct iamroot_module *m)
     g_modules[g_count++] = m;
 }
 
-size_t iamroot_module_count(void)
+size_t skeletonkey_module_count(void)
 {
     return g_count;
 }
 
-const struct iamroot_module *iamroot_module_at(size_t i)
+const struct skeletonkey_module *skeletonkey_module_at(size_t i)
 {
     if (i >= g_count) return NULL;
     return g_modules[i];
 }
 
-const struct iamroot_module *iamroot_module_find(const char *name)
+const struct skeletonkey_module *skeletonkey_module_find(const char *name)
 {
     if (name == NULL) return NULL;
     for (size_t i = 0; i < g_count; i++) {