Add --version flag support to gh-aw CLI

[Copilot]

Overview

Implements support for the --version flag to provide parity with gh CLI behavior. Previously, only the version subcommand was available, and using --version would result in an error.

Changes

Before

$ gh aw --version
Error: unknown flag: --version

After

$ gh aw --version
ℹ gh aw version v0.10.0
ℹ GitHub Agentic Workflows CLI from GitHub Next

$ gh aw version
ℹ gh aw version v0.10.0
ℹ GitHub Agentic Workflows CLI from GitHub Next

Both commands now work and produce identical output.

Implementation Details

• Added Version field to rootCmd: Leverages Cobra's built-in version flag support by setting the Version field on the root command
• Configured custom version template: Uses the same console formatting as the existing version subcommand for consistency
• Set output to stderr: Ensures all CLI output goes to stderr per project guidelines using SetOut(os.Stderr)
• Added comprehensive tests: Verifies both --version and version work correctly and produce identical output

Testing

• ✅ New tests verify --version flag functionality
• ✅ Tests ensure output consistency between flag and subcommand
• ✅ All existing tests continue to pass
• ✅ make agent-finish completes successfully

This change maintains backward compatibility while adding the requested functionality with minimal code changes.

Fixes #[issue number]

Original prompt

the cli should support the flag --version and display the version: gh aw --version. Parity with the gh behavior.

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