Add OpenCode Devcontainer Feature

[iamralch]

1. High-Level Summary

What changed: A new devcontainer feature called "opencode" has been added
to the collection, enabling automated installation of the OpenCode CLI (an
open source coding agent) in development containers.

Why it changed: This addition expands the devcontainer features collection
to include OpenCode, allowing developers to easily provision development
environments with the OpenCode coding agent pre-installed via a simple
feature declaration.

2. Detailed Technical Breakdown

2.1 Behavior & Execution Flow

When a devcontainer includes this feature, the installation script:

1. Updates apt package lists if stale
2. Installs required dependencies (ca-certificates, curl)
3. Downloads and executes the OpenCode installer from https://opencode.ai
4. The -y flag ensures non-interactive installation

2.2 Key Modifications

New files added:

• src/opencode/devcontainer-feature.json - Feature manifest defining
  metadata, version (0.0.1), and configurable options
• src/opencode/install.sh - Shell script handling dependency installation
  and OpenCode CLI setup
• src/opencode/README.md - Documentation with usage examples and options
• test/opencode/scenarios.json - Empty test scenarios configuration
• test/opencode/test.sh - Verification test checking CLI availability

CI pipeline updates:

• Added opencode to the feature matrix in both test and release jobs
  within .github/workflows/ci.yaml

2.3 Design Considerations

• Consistent patterns: The feature follows the same structure as existing
  features (atlas, starship) with identical file organization
• Minimal dependencies: Only installs ca-certificates and curl as
  prerequisites, keeping the footprint small
• Version flexibility: Supports a version option defaulting to "latest",
  though the installer script currently ignores non-latest versions
• Installation ordering: Configured to install after common-utils via
  installsAfter to ensure base utilities are available

2.4 Assumptions & Edge Cases

• Debian/Ubuntu only: Uses apt-get, limiting support to Debian-based
  distributions
• Network dependency: Requires internet access to fetch the installer
• Version parameter unused: The VERSION variable is unset when "latest"
  but the curl command doesn't pass any version parameter to the installer
• Comment inconsistency: Install script contains comment "Install Atlas
  CLI" which should read "Install OpenCode CLI"
• README typo: Example shows opecode:1 instead of opencode:1

3. Impact Assessment

Functionality:
Adds a new installable feature to the devcontainer collection, expanding
available tooling options for developers.

Performance:
No impact on existing features. Installation adds minimal overhead (two small
packages plus the OpenCode binary).

Reliability & correctness:
The test verifies basic CLI availability by checking help output. The empty
scenarios.json suggests no additional test scenarios are configured.

Security:
Executes a remote script via curl | sh pattern. Users implicitly trust
the opencode.ai domain and its HTTPS certificate chain.

Developer experience & maintainability:
Follows established patterns in the repository, making maintenance
straightforward. CI integration ensures the feature is tested against
Debian and Ubuntu base images.

Users or downstream systems:
Developers can now add OpenCode to devcontainers with a single feature
declaration, streamlining environment setup for teams using this coding agent.