feat(task): add task archive

[timimsms]

Summary

Adds cu task archive, which had no equivalent in the CLI.

$ cu task archive 86e312u87
✓ Archived task 86e312u87: …
$ cu task archive 86e312u87 --undo
✓ Unarchived task 86e312u87: …

Why archive rather than close or delete. Archiving removes a task from active views while keeping its history and its URL. For retiring a task whose work has moved elsewhere, that is the only correct primitive: closing asserts a status the task never actually reached — and after #45 we know "closed"/"complete" is not even a status name on every list — while deleting destroys the record and breaks any link to it.

Implementation note worth a look: the SDK cannot express this. clickup.TaskUpdateRequest has no archived field, so UpdateTask can never send it. The request is built directly against the SDK's exported NewRequest/Do, which is the same path its own services use, rather than adding a second HTTP client.

Verification

Round-tripped against a real workspace on a throwaway task: archived goes false → true → false, and the task's URL and status are intact afterwards. The task was deleted after the test.

Checklist

• ./scripts/ci.sh passes locally — except errcheck, which reports the same pre-existing findings on main, none in files this PR touches
• Commit messages use conventional prefixes
• CLI docs regenerated (cu docs markdown --dir docs/site/commands) and added to the mkdocs nav
• Docs updated if user-facing behavior changed — README example