Hide sub-issues from top-level issue lists

Sub-issues should only be accessible through their parent issue and must not appear alongside top-level issues.

Acceptance criteria:

• Project All view excludes sub-issues.
• Sprint, planned, backlog, search, and other top-level issue collections exclude sub-issues unless explicitly designed as child views.
• Parent issue remains canonical route for discovering and opening its sub-issues.
• Direct links to sub-issues continue to work.