feat(issues): add --estimate to issues update

[moredatarequired]

Summary

Adds --estimate and --clear-estimate options to the issues update command, allowing users to set or clear point estimates on issues.

Changes

• Added --estimate <points> option to set point estimate
• Added --clear-estimate option to clear existing estimate
• Added validation to prevent using both flags together
• Updated UpdateIssueArgs type to allow null for clearing

Usage

# Set estimate
linearis issues update ABC-123 --estimate 5

# Clear estimate
linearis issues update ABC-123 --clear-estimate

[iamfj]

Thanks for adding estimate support, @moredatarequired!

The project has been completely rewritten since this PR was opened (#49) — new architecture, GraphQL codegen, and different service patterns. This PR is unfortunately no longer compatible.

Closing this, but #26 remains open as a feature request. If you'd like to re-implement this on the new architecture, we'd welcome it — it should be straightforward following the existing issue update patterns.

Appreciate the contribution! 🙏