feat: add Kits by Kind table, zsh and brew mixins

[dangeReis]

Summary

This PR adds two new mixin kits and improves documentation discoverability:

New Mixins

• zsh: Zsh shell with default configuration

  • Installs Zsh via apt-get
  • Sets Zsh as default shell for the agent user
  • Includes basic aliases (ll, la, l)

• brew: Homebrew package manager

  • Installs Homebrew in non-interactive mode
  • Configures for Linux environment
  • Sets up shell integration

Documentation Improvements

• README.md: Added "Kits by Kind" table for better discoverability
• CONTRIBUTING.md: Added guidance on choosing the right kind for new kits

Testing

• All TCK tests pass for both new mixins
• Spec validation tests pass
• Follows existing mixin patterns

Changes

• 9 files changed, 314 insertions(+), 18 deletions(-)
• 2 new mixin kits (zsh, brew)
• Updated README and CONTRIBUTING documentation

[mdelapenya]

Thanks for this! I added a few comments, the most relevant one is regarding opencode.

I'm interested in learning about your use cases for kits and sbx: how do you work with them?

[mdelapenya]

suggestion: should this be an agent instead?

I wonder if this will collide with the existing opencode agent we distribute alongside sbx 🤔 cc/ @kiview @ejia-docker

[dvdksn]

it should be an agent kit, but I don't think we should create a separate agent spec here when opencode is already preinstalled in the opencode built-in agent.

[dangeReis]

Appologies, my agent got a little too ahead of itself submitting the PR. I'm still working on this, and I think the first thing would be creating a mixin folder to organize things better.

[dangeReis]

it should be an agent kit, but I don't think we should create a separate agent spec here when opencode is already preinstalled in the opencode built-in agent.

The goal of this mixin was to have my opencode config mounted into the sbx. Combined with the new custom secrets feature this would allow the sandbox to be started fully authenticated, yet without any secrets in the sbx, and have a config that's not necessarily the same as my local one.

[dangeReis]

Superseded by new PR with reorg, zsh, and brew only