Add machine-readable (JSON) log output to all CLI tools

[Copilot]

All CLI binaries only supported human-readable log output, with no way to consume logs programmatically.

Changes

• {aptlyctl,apt2aptly,obs2aptly,aptly-latest-snapshots}/Cargo.toml: enable the json feature for tracing-subscriber
• Each binary's main.rs:
  • Add LogFormat enum (pretty | json) with clap::ValueEnum
  • Add --log-format to Opts
  • Extract subscriber initialization into a local fn init_tracing(LogFormat) helper
  • Fix initialization order to Opts::parse() → init_tracing() → color_eyre::install() so ErrorLayer is registered before color-eyre's hook runs
• aptlyctl/src/repo.rs: fix pre-existing clippy::useless_conversion warning

Usage

aptlyctl --log-format json snapshot list
obs2aptly --log-format json my-repo ./obs-packages

JSON output uses tracing-subscriber's built-in JSON formatter; pretty (the default) preserves existing behavior.