feat(one-shot-token): add logging for cached token accesses

Add print statements whenever getenv() or secure_getenv() are called
for sensitive tokens, even when returning cached values. Previously,
only the first access was logged. Now subsequent accesses also log
"accessed (cached value)" for better debugging visibility.

This helps diagnose token access patterns and verify that the
one-shot-token library is working correctly.

Example output:
- First access: "Token GITHUB_TOKEN accessed and cached (value: ghp_...)"
- Subsequent: "Token GITHUB_TOKEN accessed (cached value: ghp_...)"

Co-authored-by: lpcox <15877973+lpcox@users.noreply.github.com>

Author: Copilot

containers/agent/one-shot-token/one-shot-token.c

@@ -290,6 +290,8 @@ char *getenv(const char *name) {
     } else {
         /* Already accessed - return cached value */
         result = token_cache[token_idx];
+        fprintf(stderr, "[one-shot-token] Token %s accessed (cached value: %s)\n", 
+                name, format_token_value(result));
     }
 
     pthread_mutex_unlock(&token_mutex);
@@ -354,6 +356,8 @@ char *secure_getenv(const char *name) {
     } else {
         /* Already accessed - return cached value */
         result = token_cache[token_idx];
+        fprintf(stderr, "[one-shot-token] Token %s accessed (cached value: %s) (via secure_getenv)\n", 
+                name, format_token_value(result));
     }
 
     pthread_mutex_unlock(&token_mutex);