Feature request: `basecamp todos update` command

[brigleb]

The CLI currently supports update for messages, cards, files, and docs — but not for todos. The only way to update a todo's title, description, assignees, or due date is through basecamp api put, which requires knowing the raw API path and building JSON payloads by hand.

Proposed syntax (matching the existing patterns from messages update and cards update):

basecamp todos update <id|url> [flags]

Flags:
  -t, --title string      New title (maps to API field "content")
  -b, --body string       New description/notes body
  -d, --due string        Due date (natural language or YYYY-MM-DD)
      --starts string     Start date
      --assignee string   Assignee ID or name (repeatable)

Examples:

basecamp todos update 9678557587 --title "Updated todo title"
basecamp todos update https://3.basecamp.com/.../todos/123 --body "<div>New description</div>"
basecamp todos update 123 --due "next Friday" --assignee "Kandace"

Why it matters: I need to “clarify” tasks a lot, particularly with an agent. Today the workaround is basecamp api put /buckets/{id}/todos/{id}.json -d '{"content":"...", "description":"..."}', which is cumbersome and error-prone compared to the ergonomic flag-based interface other resources already have.