feat: Extract shell configuration into a devcontainer feature

Problem

Shell configuration is currently baked into the template Dockerfile via chsh. This:

• Requires a custom Dockerfile even when no other customization is needed
• Mixes concerns (shell setup vs. other image customization)
• Cannot be easily swapped or disabled

Proposal

Create a devcontainer feature for shell configuration that:

1. Runs chsh to set the default shell for the vscode user
2. Configures VS Code terminal profiles via feature customizations
3. Accepts a shell option (fish, zsh, bash, etc.)

Example Usage

{
  "features": {
    "ghcr.io/technicalpickles/devcontainer-features/shell:1.0": {
      "shell": "fish"
    }
  }
}

Benefits

• Template Dockerfile becomes FROM base only (or eliminated entirely)
• Shell choice is explicit and composable
• Easier to support multiple shells
• Follows devcontainer feature conventions

Related

This was identified during the persistent home volume design work. See docs/plans/2025-01-08-persistent-home-volume-design.md.

Read docs/plans/2025-01-08-persistent-home-volume-design.md, then inspect how shell configuration is currently handled in the template Dockerfile. Define the feature's shell option, chsh behavior, and VS Code terminal customizations, and verify that the feature works with the example fish configuration and other supported shells.