fix: register custom tasks in task list/show/run commands

[orban]

Summary

Custom tasks defined under tasks.custom in config were parsed and validated correctly, but only registered at runtime by the run and preview commands. The task list, task show, and task run subcommands called tasks.AllDefinitionsSorted() / tasks.GetDefinition() without first registering custom tasks, so they only saw built-in tasks.

Fix

Added a registerCustomTasks() helper in task.go that loads config and calls RegisterCustomTasksFromConfig — the same pattern already used in run.go:228-230 and preview.go. Called at the top of runTaskList, runTaskShow, and runTaskRun.

Before/After

# Before
$ nightshift task list | grep custom
(nothing)

$ nightshift task show my-custom-task
Error: unknown task: my-custom-task

# After
$ nightshift task list | grep custom
my-custom-task [custom]  My Custom Task  Analysis  Low  10k-50k  Low

$ nightshift task show my-custom-task
Task:    My Custom Task
Type:    my-custom-task
Custom:  yes
...

Test plan

• Existing TestTaskInstanceFromDef passes
• Existing TestRegisterCustom_* tests pass
• Manual: task list shows custom tasks with [custom] label
• Manual: task show <custom> displays details and prompt
• Manual: task run <custom> --dry-run shows correct prompt