feat: --remind-at sets Linear's issue reminder on create and update

[muqsitnawaz]

Refs PHNX-4269 (AGI Menu). The owner asked to set an exact time on a to-do ("I want to be able to select the exact time… increments of 10").

Why a reminder

• The due date can't hold a time. Linear's Issue.dueDate is a TimelessDate (date only).
• The reminder can. The schema has issueReminder(id, reminderAt: DateTime), Linear's "Remind me", which notifies the user at an exact moment. That's where a to-do's time belongs, with the date still in dueDate.

Change

• One new option: linear create … --remind-at <time> and linear update <ID> --remind-at <time>.
• Accepted input: YYYY-MM-DDTHH:MM (a space works in place of the T), with seconds and a zone optional. No zone means local time, and it's sent to Linear as UTC.
• Refused input: a date with no time, or text that isn't a time, is refused before anything is created.
• create: sets the reminder on the new issue's identifier, and exits 1 if that step fails.
• update: sets it on the issue it resolved.

Verification

• python3 -m unittest test_linear: 231 tests, OK (4 skipped). New RemindAtTest: zoned times go out as UTC, local and space-separated input both parse, and a date-only value or junk text is refused.
• Live check of the mutation, with nothing written: I sent the mutation for an issue that doesn't exist (PHNX-999999), and Linear answered:

  {"data": null, "errors": [{"message": "Entity not found: Issue", "code": "INPUT_ERROR"}]}
  

  That's a not-found error rather than a GraphQL validation error, so the mutation's shape and types are accepted, and an identifier is accepted as the id.
• Not done: I didn't set a real reminder, which would have sent the owner a notification.

Docs

• A skill.md example and a CHANGELOG entry under Unreleased.

🤖 Generated with Claude Code

[muqsitnawaz]

Independent review (non-author subagent): APPROVE, no blocking findings.

The reviewer ran RemindAtTest and drove parse_remind_at under both UTC and America/Los_Angeles:

• Zoned input converts correctly, for example +05:30 gives 03:40Z.
• Z, a space separator, seconds and microseconds all work.
• Date-only, 09:10, Tomorrow and empty input are refused.
• The DST gap and DST overlap hours resolve sensibly.
• create and update have no --json mode, so there's no output contract to break.

All three of its suggestions are applied in the follow-up commit:

1. create refuses --remind-at together with --from-file, instead of silently dropping it.
2. update parses --remind-at once, before any change is made. A typo can no longer leave a half-applied update or exit partway through a batch.
3. create now requests the new issue's id and passes it to issueReminder, as update does. A failed reminder prints the exact retry command, linear update <ID> --remind-at ….

Re-run after the follow-ups: python3 -m unittest test_linear, 231 tests OK.