smitebot: implement smitebot stop command

[Ashish-Kumar-Dash]

Summary

Adds smitebot stop <campaign-id>: tears down a running campaign and reaps all of its processes, including the Nyx QEMU children that a plain tmux kill-session leaks.

The problem

In Nyx mode each afl-fuzz runner spawns a QEMU child, and tmux kill-session only sends SIGHUP to the afl-fuzz panes, the QEMU children orphan (reparented away from tmux) and keep running.

How it works

Verified against the live process tree and AFL++ source:

• Each afl-fuzz pane is its own process-group leader. Its QEMU child is in the same process group and keeps that group id even after it orphans. So one group-directed kill reaps the QEMU too.
• In testing, an orphaned QEMU did not exit on SIGTERM, so the kill escalates SIGTERM → SIGKILL.
• A killed afl-fuzz lingers as a zombie under tmux remain-on-exit until the session is torn down, so the "did everything die?" check ignores zombie (Z) states.

stop:

1. Loads state.json for the campaign (errors name the path + how to list campaigns).
2. Reads the live runner PIDs from the tmux session (tmux list-panes) — not from state.json, whose PIDs can be missing (slow startup) or stale/recycled (after a reboot), killing a recycled PID's group could hit an unrelated process.
3. pkill -TERM -g <pgid> each group → waits (polling, non-zombie only) → pkill -KILL -g <pgid> any straggler.
4. tmux kill-session to remove windows and reap the zombie leaders.
5. Records status = "stopped" + stop_time in state.json.

state.json changes

• New status stopped; new field stop_time: Option<u64> (#[serde(default)], so pre-existing state files still load).

Live end-to-end test I ran (2-runner lnd campaign):

Before — 2 afl-fuzz, their 2 QEMU, 1 session:

$ pgrep -a afl-fuzz ; pgrep -a qemu ; tmux ls
12122 .../afl-fuzz -Y ... -M 0 ...
12128 .../afl-fuzz -Y ... -S 1 ...
12131 .../qemu-system-x86_64 ... worker_id=0 ... load=off
12158 .../qemu-system-x86_64 ... worker_id=1 ... load=on
lnd-encrypted_bytes-1783687998: 2 windows

Stop, then re-check — zero survivors:

$ smitebot stop lnd-encrypted_bytes-1783687998
INFO  [smitebot::commands::stop] campaign lnd-encrypted_bytes-1783687998
stopped

$ pgrep -a afl-fuzz ; pgrep -a qemu ; tmux ls
no server running on /tmp/tmux-1000/default

$ grep -E 'status|stop_time'
~/.smitebot/runs/lnd-encrypted_bytes-1783687998/state.json
  "status": "stopped",
  "stop_time": 1783688225,

Ref. #70