refactor(agnocast_kmod): split agnocast_main.c into multiple files

[Koichi98]

agnocast_main.c has grown to ~3500 lines and mixes several concerns, making it hard to navigate and maintain.

Proposed changes

• Split into multiple files by responsibility (e.g., topic operations, exit handling, ioctl dispatch)
• Refactor the ~590-line agnocast_ioctl() dispatch function by extracting helpers for repeated boilerplate and splitting into sub-functions by category
• Any other cleanup opportunities found along the way

[k1832]

I see more refactor opportunities in the kernel module code (e.g. remove unnecessary uses of ternary operators, replace cleanups with conventional go-to styles). I might file a new issue for this.

With all that, this issue itself and the PR looks fantastic. Thank you so much for the effort.

[Koichi98]

Note: Reduce forward declaration in the kernel module. Just have to reorder the functions to fix this.

@k1832
Could you add the above point once you've created the new issue?

[k1832]

@Koichi98 Sure! Thanks for pointing that out