feat: add github-cli to Dockerfile as explicit dependency

[avoidwork]

Description

Add the GitHub CLI (gh) to the Dockerfile as an explicit package dependency in the runtime stage. The Alpine package github-cli provides the gh binary, ensuring it is present in every built container image.

Type of Change

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Refactor (no functional changes)
• Performance improvement
• CI / build / tooling

Testing

• Docker image builds successfully with github-cli included
• gh --version returns v2.93.0 inside the container
• No code changes to src/, index.js, or skill scripts
• npm start runs without crashes (TUI raw mode error is expected in non-interactive environments)

Coverage

• N/A — no code changes, only Dockerfile dependency addition and OpenSpec artifacts

Checklist

• npm run lint passes
• Tests pass with 100% line coverage
• No forbidden patterns used
• Conventional Commit style applied

Files Changed

• Dockerfile — added github-cli to apk add --no-cache line in runtime stage
• OpenSpec artifacts (proposal, design, specs, tasks) for the change

[avoidwork]

Audit Results: add-gh-cli-dockerfile-dependency

Step 5: Spec Audit — PASS (0 errors)

• Coverage: All goals covered by specs and tasks
• Fidelity: Specs faithfully represent original intent
• Completeness: No missing requirements or edge cases
• Consistency: Tasks map cleanly to spec requirements

Step 9: Implementation Audit — PASS (0 errors)

Goal Fulfillment

• Goal 1: Add gh CLI as explicit package dependency — ✅ COMPLETE
  • github-cli (Alpine package for gh CLI) added to Dockerfile runtime stage apk add line
  • Package is in the correct position (between git and file)
  • Dockerfile syntax is valid
  • Docker image builds successfully
  • gh --version works inside the container (v2.93.0)

Spec Compliance

• Requirement: "gh CLI must be installed in the container image" — ✅ SATISFIED
  • github-cli is installed via apk add --no-cache in the runtime stage
  • The gh binary is present and functional in every built image

Task Completion

• 1.1: Add gh to the apk add line — ✅ COMPLETE (used github-cli Alpine package)
• 1.2: Verify Dockerfile syntax — ✅ COMPLETE (valid syntax, no issues)
• 1.3: Build image and verify gh --version — ✅ COMPLETE (v2.93.0)

Quality Check

• Dockerfile change: Single-line modification, clean and minimal
• Package selection: github-cli is the correct Alpine package name for the gh CLI
• No code changes: As specified, no changes to src/, index.js, or skill scripts
• Application startup: npm start runs without crashes (TUI raw mode error is expected in non-interactive environments)

Notes

• The Alpine package is named github-cli, not gh. The github-cli package provides the gh binary.
• The npm start error about raw mode is from the Ink TUI framework and is expected when running without a proper TTY. This is not a flaw in the implementation.

[avoidwork]

Fixes #449