Add --version flag to print build info

[irubey]

Summary

Adds a --version flag to the genqlient CLI that displays version information including commit hash and build date.

Changes

• Added --version flag to generate/cliArgs struct
• Implemented printVersion() function that uses runtime/debug.BuildInfo to extract:
  • Module version (or "dev" for development builds)
  • Git commit hash (shortened to 12 characters)
  • Build timestamp
• Updated changelog with new feature entry

Example Output

For development builds:

genqlient dev (8ddeeee0a15a, built 2025-10-28T05:47:17Z)

For release builds:

genqlient v1.2.3 (abc123def456, built 2025-10-28T05:47:17Z)

Testing

• ✅ All existing tests pass
• ✅ Lint passes (make lint)
• ✅ Manually tested with go run . --version and go build + ./genqlient --version
• ✅ Help output correctly shows the new flag

Notes

The version flag follows common CLI conventions and provides useful debugging information. Pseudo-versions (like v0.0.0-...) are normalized to "dev" for cleaner output, while real version tags display normally.

Issue #296

I have:

• Written a clear PR title and description (above)
• Signed the Khan Academy CLA
• Added tests covering my changes, if applicable
• Included a link to the issue fixed, if applicable
• Included documentation, for new features
• Added an entry to the changelog

[csilvers]

This makes sense to me! I'm approving, but @benjaminjkraft may have feedback he wants to give too (like other version info to report, or some such). But I think it's fine to land this as-is and iterate if need be.

[benjaminjkraft]

Indeed, thanks!