feat: add custom view management commands

[eslerm]

Add CRUD for saved custom views (filtered issue/initiative/project views).

Scope

• 4 queries: customView, customViews, customViewDetailsSuggestion, customViewHasSubscribers
• 3 mutations: customViewCreate, customViewUpdate, customViewDelete
• New command group with 5 subcommands
• Filter config is JSONObject -- CLI accepts raw JSON or file path
• Estimated ~1,140 LOC

Open Questions

• Accept --filter-data as inline JSON, file path, or both?
• Implement AI suggestion endpoint (customViewDetailsSuggestion) or skip?
• How to handle view name ambiguity? (by creator? team?)

See planning/02-custom-views.md