feat: add --due-date flag to create and update

[muqsitnawaz]

Summary

• linear create --due-date YYYY-MM-DD sets the due date at creation time.
• linear update <id> --due-date YYYY-MM-DD sets it on existing issues; --due-date none clears it.
• Uses Linear's TimelessDate (date-only) field on IssueUpdateInput.dueDate / IssueCreateInput.dueDate.
• Validation via datetime.date.fromisoformat — stdlib only, zero new dependencies.

Why

Linear's web UI and the official @linear/cli both expose due dates as a first-class field. linear-cli previously fetched and sorted by dueDate but had no way to set one — so users had to fall through to GraphQL by hand. Closes that gap with two flags and one helper.

Verified end-to-end against the Linear API

```
$ linear update RUSH-565 --due-date 2026-04-26
RUSH-565 -> due: 2026-04-26

$ linear update RUSH-565 --due-date none
RUSH-565 -> due: cleared

$ linear update RUSH-565 --due-date not-a-date
Invalid --due-date 'not-a-date'. Use YYYY-MM-DD (e.g. 2026-04-28) or 'none'.

$ linear create "TEST" --due-date 2026-12-31
Created RUSH-566: TEST [no cycle | Muqsit]

(verified dueDate=2026-12-31 via GraphQL, then deleted)

```

Test plan

• Create with valid date sets dueDate (verified via GraphQL read-back)
• Update with valid date sets dueDate
• Update with none clears dueDate
• Invalid date string exits non-zero with a clear message
• Help text lists --due-date for both subcommands
• Existing flows untouched (no other args/handlers changed)