[feature]: Sub-issue sequence IDs should reflect parent-child hierarchy

Summary:
The current work item numbering uses flat sequential IDs (e.g., parent=PROJ-5, sub=PROJ-6) which visually flatten the hierarchy. When viewing a list, it's not immediately clear which items are sub-issues of which parent. This is especially confusing in board/list views where sequential ordering doesn't map to parent-child depth.

Proposed solution:
Adopt a hierarchical ID format where sub-issues carry their parent's sequence as a prefix. For example:

• Parent: PROJ-5
• Sub-issues: PROJ-5.1, PROJ-5.2, PROJ-5.3

Alternatively (simpler): hide or visually indent sub-issue sequence IDs in list/board views, since sub-issues are already grouped under their parent in the tree view — the flat IDs add noise without conveying hierarchy.

Why this should be worked on:
Users working with deeply nested issues lose spatial awareness of the hierarchy. Flat IDs force users to cross-reference the tree view, slowing down navigation. This is a UX friction point that compounds as projects grow. Many project management tools (Jira, Linear, etc.) use hierarchical numbering or suppress sub-issue IDs for this exact reason.

The issue names no files, tests, or entry points. First clarify whether hierarchical IDs or hiding/indenting IDs is the intended solution, then trace sequence-ID generation and rendering in board and list views. Done means the chosen behavior consistently communicates parent-child hierarchy without breaking existing issue references.