Implement OpenSpec change tasks

Work through a change's task checklist against real code, one task at a time, checking each off only once it is actually done.

Steps

1. Select the change. Use a given name; otherwise infer from context, or auto-select if only one active change exists. If ambiguous, run openspec list --json and let the user choose. Always announce which change is in use.

2. Check status: openspec status --change "<name>" --json — read schemaName and which artifact holds the tasks.

3. Get apply instructions:

   openspec instructions apply --change "<name>" --json