Child threads show their parent in the header again #201

What: A child thread's header shows the parent thread's title (linking back) before its own title, with a menu to switch between sibling threads, as in v1.
Why: The V2 port dropped the fork's header crumbs (v1 #18); V2 only shows "Subagent of" at the top of the conversation, which scrolls away.
So what: Implemented in the linked PR; the user approves the merge.

Contribution to the Objective

Drafters start as children; the user needs to see and move between a child and its parent while directing them.

Acceptance criteria

• Header of a thread whose lineage has a parent (relationship subagent, not fork) shows project / parent title / thread title; the parent title opens the parent.
• When the parent has other live children in the same environment, a compact menu next to the parent title lists them, oldest first.
• Top-level threads and forks look unchanged; only child threads subscribe to the full thread list.

Non-goals

Mobile header; multi-level ancestry (grandparents).

Blockers

None.

Proof

Focused logic tests; web typecheck and lint.

Elon record

Requirements and who asked: User asked to bring back the stacked parent title for child threads.
Deleted: Grandparent chains, mobile, the v1 Agents panel tree.
Bottleneck: None; the V2 thread shell already carries lineage.
Checked myself: v1 ThreadParentCrumbs (ffcdae8) and V2 ChatHeader/lineage sources.