feat(FN-760): use source task labels for refinement titles

- Derive readable refinement titles from source task title or first line of description instead of raw task ID
- Fall back to task ID only when both title and description are empty/whitespace
- Collapse internal whitespace in description-derived labels for clean output
- Update PROMPT.md heading to match the new readable refinement title
- Add comprehensive tests for title precedence, whitespace handling, and edge cases
- Document refinement naming behavior in README
- Add changeset for @gsxdsm/fusion patch release

README.md

@@ -145,6 +145,8 @@ fn task pause FN-001                     # Pause automation for task
 fn task unpause FN-001                   # Resume automation for task
 ```
 
+**Refinement tasks** created with `fn task refine` inherit a readable label from the source task. The title follows the format `Refinement: {source label}` where the source label is the original task's title (if set), otherwise the first line of the original description. This keeps refinement task names meaningful even when the source task has no explicit title.
+
 **GitHub Integration:**
 ```bash
 fn task import owner/repo                  # Import all open issues