[codex-analytics] emit forked thread id on initialization

[kbazzi]

Why

• Thread initialization analytics do not identify the source thread for forked threads.
• The session viewer needs this lineage to construct thread trees.
• Depends on openai/openai#987854. Do not release this change before that backend schema change is deployed.

What Changed

• Adds optional forked_from_thread_id to codex_thread_initialized.
• Populates it from the existing thread fork lineage for app-server and in-process subagent initialization paths.
• Keeps it null for non-forked threads.

Verification

• just fmt
• just test -p codex-analytics
• just test -p codex-app-server thread_fork_tracks_thread_initialized_analytics

[github-actions]

All contributors have signed the CLA ✍️ ✅
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[kbazzi]

I have read the CLA Document and I hereby sign the CLA

[owenlin0]

nice!