Feature: Add due date support

[g-click-trade]

Issue co-written with dear assistant Claude Code 🤖

Feature Request

Add support for reading and setting due dates on issues.

Current Behavior

linearis issues read does not return the dueDate field, and there's no way to set due dates via CLI.

Proposed Solution

1. Read: Include dueDate field in issues read output
2. Create: Add --due-date <date> option to issues create
3. Update: Add --due-date <date> option to issues update
4. Clear: Add --clear-due-date option to issues update

Example Usage

# Create with due date
linearis issues create "Urgent fix" --team ENG --due-date 2025-01-15

# Update due date
linearis issues update ENG-123 --due-date 2025-02-01

# Clear due date
linearis issues update ENG-123 --clear-due-date

# Read due date
linearis issues read ENG-123 | jq '.dueDate'
# "2025-02-01"

Date Format

ISO 8601 format (YYYY-MM-DD) would be consistent and unambiguous.

[Hallicopter]

Bumping this up! Will probably add a PR for this -- super useful!