chore: Add agentic coding tooling

.crosslink/rules/c.md

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+### C Best Practices
+
+#### Memory Safety
+- Always check return values of malloc/calloc
+- Free all allocated memory (use tools like valgrind)
+- Initialize all variables before use
+- Use sizeof() with the variable, not the type
+
+```c
+// GOOD: Safe memory allocation
+int *arr = malloc(n * sizeof(*arr));
+if (arr == NULL) {
+    return -1;  // Handle allocation failure
+}
+// ... use arr ...
+free(arr);
+
+// BAD: Unchecked allocation
+int *arr = malloc(n * sizeof(int));
+arr[0] = 1;  // Crash if malloc failed
+```
+
+#### Buffer Safety
+- Always bounds-check array access
+- Use `strncpy`/`snprintf` instead of `strcpy`/`sprintf`
+- Validate string lengths before copying
+
+```c
+// GOOD: Safe string copy
+char dest[64];
+strncpy(dest, src, sizeof(dest) - 1);
+dest[sizeof(dest) - 1] = '\0';
+
+// BAD: Buffer overflow risk
+char dest[64];
+strcpy(dest, src);  // No bounds check
+```
+
+#### Security
+- Never use `gets()` (use `fgets()`)
+- Validate all external input
+- Use constant-time comparison for secrets
+- Avoid integer overflow in size calculations