chore: migrate container runtime from Colima/Docker to Podman

[iheitlager]

Description

Migrate the local development container runtime from Colima + Docker to Podman,
with optional k3s/Rancher Desktop for Kubernetes-lite testing. This covers
dotfiles changes: brew packages, shell aliases, and environment setup.

Complexity

Estimate: small
Reasoning: Mostly configuration and alias changes; docker-compose → podman-compose
is largely drop-in. K8S variant is additive and optional.

Context

Currently using Colima as a Docker runtime host on macOS. Podman offers:

• Rootless containers (no daemon, better security)
• Drop-in docker alias compatibility
• podman-compose for existing compose projects
• Option to layer in Rancher Desktop for k3s (K8S lite)

Acceptance Criteria

• podman and podman-compose added to bash/brew_packages
• docker aliased to podman in shell env (with note about compose)
• DOCKER_HOST / socket env vars configured if needed for compose compat
• Colima removed or made optional in brew packages
• Optional: Rancher Desktop cask added with k3s notes
• Tested against existing compose-based projects

Additional Notes

• Rancher Desktop (rancher) is a Homebrew cask — conflicts with standalone Podman if both manage the VM
• podman machine replaces Colima's VM on macOS
• Consider documenting the migration in docs/colima_to_podman.md (only if explicitly requested)