termux-job-scheduler: New jobs default to job ID 0

Problem description

When creating a new job, with no ID given it assumes ID 0 and replaces an existing job of that ID.

Steps to reproduce

$ termux-job-scheduler -s ~/jobs/one.sh --period-ms 14400000
Scheduling Job 0: /data/data/com.termux/files/home/jobs/one.sh       (periodic: 14400000ms) ... - response 1

$ termux-job-scheduler -s ~/jobs/two.sh --period-ms 14400000
Scheduling Job 0: /data/data/com.termux/files/home/jobs/two.sh       (periodic: 14400000ms) ... - response 1

Expected behavior

I'd expect the default behaviour to choose the next available job number and not replace an existing job.

  --job-id int               job id (will overwrite any previous job with the same id)

This reads to me like it will override a job only if you explicitly give it an existing job ID (it doesn't specify a default of 0).

Additional information

$ termux-info
Termux Variables:
TERMUX_APP_PACKAGE_MANAGER=apt
TERMUX_MAIN_PACKAGE_FORMAT=debian
TERMUX_VERSION=0.118.0
Packages CPU architecture:
aarch64
Subscribed repositories:
# sources.list
deb https://grimler.se/termux/termux-main stable main
Updatable packages:
All packages up to date
termux-tools version:
1.31.0
Android version:
12
Kernel build information:
Linux localhost 4.19.157-perf+ #1 SMP PREEMPT Sat Sep 17 00:13:28 CST 2022 aarch64 Android
Device manufacturer:
OnePlus
Device model:
EB2103

Start with the termux-job-scheduler command entry point and reproduce the two scheduling commands shown in the issue. Trace how an omitted --job-id is handled and verify the behavior when multiple jobs are created without explicit IDs. Done means a new job receives the next available ID instead of replacing job 0, while an explicitly supplied existing ID still overwrites that job.