Add child-thread ownership and hierarchy

[VolkerChristian]

Summary

• index every child thread directly to its parent thread and owning agent while retaining ordered children per parent
• preserve ownership through hydration, live updates, reconnects, completion, rebinding, and authoritative removal
• route child lifecycle and result changes through the ownership index instead of repeated global scans
• render arbitrarily nested child threads with selection, navigation, and collapsed-by-default expansion
• align hierarchy controls and Agent presentation with the canonical visual style

Verification

• focused presentation-pipeline and shell-integration tests: 2/2 passed
• complete CodexUI suite: 7/7 passed
• exact production-shaped app-server replay preserves root ownership and resolves completed child status correctly
• live CodexUI acceptance confirmed nested navigation and completed Agent status

Reviews

• lifecycle/data-structure review: APPROVE
• hierarchy/UI review: APPROVE
• native Claude final ownership/dataflow review: APPROVE

Scope

No protocol changes. Conversation-card composition, Git/filesystem-watch/Inspector/settings optimization, and unrelated helper consolidation remain outside this PR.