Orchestrator button on worker page links to wrong parent session

[i-trytoohard]

Bug

On any worker session page (e.g. /sessions/ao-8), clicking the "Orchestrator" button in the top-right navigates to the wrong orchestrator (e.g. ao-orchestrator-1) instead of the correct parent (ao-orchestrator-9).

Root Cause

Two-part failure:

1. No parent tracking. Sessions have no parentOrchestratorId field — neither in the Session type nor in metadata. When orchestrator-9 spawns worker ao-8, that relationship is never recorded.

2. Fallback picks wrong orchestrator. When the frontend fetches /api/sessions?project=agent-orchestrator&orchestratorOnly=true:

   • The API returns all orchestrator sessions for the project sorted lexicographically by ID
   • orchestratorId is set to null when there are multiple orchestrators (sessions/route.ts:36)
   • The client fallback (page.tsx:162) does .find() which returns the first match — ao-orchestrator-1 (alphabetically first), not the active ao-orchestrator-9

Fix

Apply both approaches:

A — Store parent orchestrator in session metadata

• When an orchestrator spawns a worker, write parentOrchestratorId to the worker metadata (in session-manager spawn flow)
• Frontend reads session.metadata["parentOrchestratorId"] directly — no API fallback needed for the correct parent

B — Smart fallback when parent metadata is missing

• Sort orchestrators by recency (prefer non-exited, then by createdAt desc) instead of lexicographic
• This fixes the common case for sessions created before the metadata fix

Affected files:

• packages/core/src/session-manager.ts — write parentOrchestratorId metadata on worker spawn
• packages/web/src/lib/serialize.ts:95 — sort by recency, not lexicographic
• packages/web/src/app/sessions/[id]/page.tsx:160-163 — read parent from metadata, fallback to smart sort
• packages/web/src/app/api/sessions/route.ts:36 — return per-project orchestrator ID properly