butter tasks update is advertised in help but has no dispatcher branch

[ryanlitalien]

butter tasks update is listed in the help text but has no dispatcher branch, so it falls through to the unknown-subcommand error. Same class as #6.

Current behaviour

$ butter tasks update 42 --state done
Unknown subcommand for tasks: update

Why

The help advertises it:

tasks list | create | update       Manage tasks, bugs, and backlog

The dispatcher only handles list and create. There is no tasksUpdate function. The backend already supports PATCH /api/v1/projects/:project_id/tasks/:id.

Fix

Add tasksUpdate, mirroring tasksCreate:

• PATCH /api/v1/projects/:project_id/tasks/:id with the changed fields only
• flags: --state, --priority, --type, --title, --assignee, matching whatever tasks create accepts
• usage error when the id is missing or no field flag is given
• plain and --json output
• if (subcmd === "update") return tasksUpdate(args); in the dispatcher
• tests in the style of the projects show tests: happy path, --json, missing id, no fields, 404

Done when

butter tasks update <id> --state done --project <id> changes the task and prints the result, and npm test covers it.