feat: expand Sprint resource to match documented API

[steveclarke]

Summary

The Superthread API now documents several Sprint endpoints that weren't available when we first implemented the gem. Our current Sprint support is read-only (list and find). The API now offers full CRUD plus settings management.

Missing Endpoints

Priority	Method	Path	Description
High	POST	/{team_id}/sprints	Create sprint (project_id required, optional title, content)
High	PATCH	/{team_id}/sprints/{sprint_id}	Update sprint (title, content, state, start_date, end_date)
Low	GET	/{team_id}/sprints/settings	Get sprint settings for a space
Low	PUT	/{team_id}/sprints/settings	Update sprint settings (length, cooldown, start day, enabled, layout)

Sprint Model Gaps

The model is also missing fields the API returns. These should be added regardless of new endpoints — sprints get will immediately show more useful info:

• content (description)
• state (planned / active / completed / canceled)
• end_date
• override_end_date
• layout (board / list / timeline / calendar)
• project_id
• unfinished_cards
• progress
• user_updated

Suggested Approach

1. Update the Sprint model with missing fields (quick win)
2. Add create and update to Resources::Sprints + CLI commands
3. Sprint settings endpoints can be deferred — admin/config operations most users do in the web UI

API Docs

• Create a Sprint
• Update a Sprint
• Get Sprint Settings
• Update Sprint Settings
• Full API index