fix(server): retain events while opening a thread

[rynfar]

Opening a thread can miss events published while its initial snapshot loads, making a running agent appear stalled. Start the live subscription consumer immediately, before reading the snapshot.

Adopts upstream #9521 (9c9ae3dc0e). Applies to every provider and client connection mode without changing Pylon's admission or Prime recovery logic. Records the partial adoption in the upstream ledger.

Validation: all 154 server-router tests, server typecheck, formatting and targeted lint pass. The regression fails with the original startup and passes with the fix; it waits on the synchronization marker instead of a sleep.

CI exposed a Pylon legacy-client fixture that assumed the old subscription scheduling. Its live events now wait for the client synchronization receipt, preserving the rollback-filter assertion without timing assumptions.

The durable implementation and review record is consolidated in companion #279.

Model: GPT-6 Astra. Harness: Codex.

[github-actions]

Thread transfer impact

✅ Thread transfer remains within every enforced ceiling.

Provider	Metric	Main baseline	This PR	Impact	PR ceiling
Codex	Total thread wire	13.6 KiB	13.6 KiB	+6 B (+0.0%)	15.1 KiB	✅
Codex	Thread snapshot wire	7.1 KiB	7.1 KiB	−1 B (−0.0%)	7.3 KiB	✅
Codex	Live turn WebSocket wire	6.5 KiB	6.5 KiB	+7 B (+0.1%)	7.8 KiB	✅
Codex	Live turn WebSocket decoded	56.5 KiB	56.5 KiB	0 B (0.0%)	66.4 KiB	✅
Codex	Live turn messages	8	8	0 (0.0%)	21	✅
Claude	Total thread wire	13.6 KiB	13.6 KiB	−13 B (−0.1%)	15.1 KiB	✅
Claude	Thread snapshot wire	7.1 KiB	7.1 KiB	−8 B (−0.1%)	7.3 KiB	✅
Claude	Live turn WebSocket wire	6.6 KiB	6.6 KiB	−5 B (−0.1%)	7.8 KiB	✅
Claude	Live turn WebSocket decoded	57.3 KiB	57.3 KiB	0 B (0.0%)	66.4 KiB	✅
Claude	Live turn messages	8	8	0 (0.0%)	21	✅

Baseline: de8d090 · PR result: 9f6b117 · Source CI: success

Scenario and decoded snapshot size

10 historical turns, 5 command tools per turn, 878.9 KiB retained MCP result per historical turn, and a 1.05 MiB retained result in the measured turn.

• Codex decoded thread snapshot: 111.2 KiB
• Claude decoded thread snapshot: 111.9 KiB

Updated in place by a trusted workflow. PR artifacts are strictly validated and never executed.