feat: suppress already-installed output for apt, homebrew, and mise

[jiminu]

Summary

When running selfishell update (or update --tools-only, install), packages and tools that are already installed previously printed verbose status lines on every run:

• Apt: printed Already installed apt packages (10): ... and Already installed apt packages (2): ...
• Homebrew: printed Already installed Homebrew formula/cask (N): ...
• mise: printed mise <tool>@<version> ⇢ already installed for each configured runtime

In accordance with Selfishell's UX principle of suppressing unchanged/already-up-to-date output (similar to Neovim plugins, direct dependencies, and managed files/links), this PR silences these messages when everything is already installed:

1. Apt / Homebrew:

   • Removed Already installed apt packages and Already installed Homebrew console outputs and unused tracking arrays.
   • When all packages are already installed, they complete silently without terminal noise. Missing packages and warnings/errors continue to be reported normally.

2. mise:

   • Added a native preflight check via mise -q install --dry-run-code "$@".
   • If all tools are already installed (exit 0), install_mise_tools returns immediately without printing ⇢ already installed lines.
   • If the preflight returns non-zero, it falls through to the regular mise install, which either installs missing tools or surfaces the real error.

Verification

• Added test_apt_suppresses_already_installed_output and test_homebrew_suppresses_already_installed_output in tests/package_adapters_test.bash
• Enhanced test_installs_declared_mise_tools_with_managed_config in tests/installers_test.bash to verify the preflight-to-fallback execution sequence when tools need installing
• Added test_skips_mise_install_when_tools_are_already_installed in tests/installers_test.bash to verify preflight success skips the install phase
• Ran full repository check gate (bash scripts/check.sh) locally: syntax, shellcheck, shfmt, and all test suites pass with 0 failures.