Migrate legacy shell functionality into Rust and remove uki-setup.sh

[GlitchSlayed]

Motivation

• Consolidate the remaining behavior from the legacy uki-setup.sh into the Rust CLI so the project is maintained in one codebase.
• Preserve the shell flow: cmdline detection/sanitization, installed-kernel discovery, and stale-UKI pruning while improving safety and testability in Rust.

Description

• Add a new kernel module (src/kernel.rs) implementing list_installed_kernels, sanitize_cmdline, resolve_cmdline, and prune_stale_uki_artifacts to mirror the legacy script logic in Rust.
• Wire cmdline resolution and stale-UKI pruning into the Rust pipeline by using resolve_cmdline in generate and invoking prune_stale_uki_artifacts during install in src/app.rs.
• Extend configuration with configured_cmdline and auto_detect_cmdline (defaults and TOML parsing) in src/config.rs and surface them in the README config reference.
• Update integration tests (tests/integration_cli.rs) to cover cmdline sanitization/resolution and stale-UKI pruning, add src/kernel.rs, and remove the legacy uki-setup.sh and its shell test harness (tests/test_uki_setup.sh).

Testing

• Ran cargo fmt to format the code successfully.
• Ran cargo test and all tests passed (unit + integration suites completed successfully).

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Codex Task