fix(server): use T3 sender for Claude userEmail (TEC-1031)

[tusharbhardwaj-bk]

Claude's native system prompt currently reports the shared Claude subscription email, so agents can misattribute the person sending a T3 Code message.\n\nThis keeps the native Claude Code prompt and appends T3's resolved message-sender email as the authoritative userEmail. If T3 cannot resolve a sender, the prompt explicitly keeps the user unknown instead of falling back to account, OS, or Git identity. Non-T3 Claude sessions remain unchanged.\n\nValidation:\n- Focused Claude identity and adapter tests: 76 passed\n- Server-scoped typecheck: passed\n- Changed-file format and lint: passed (three pre-existing ClaudeAdapter unused-import warnings)\n- Fork-marker check: passed\n- Full PR CI: 10 successful checks, 0 failures\n- Staging PR #128 merged to expbkmain\n- expbkt3 deployed SHA: 817b418\n- t3-expbkt3.service: active; private health request: HTTP 200\n\nTEC-1031\n\nImplemented with GPT-5.6 SOL in the Codex harness.

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[github-actions]

Thread transfer impact

✅ Thread transfer remains within every enforced ceiling.

ℹ️ No successful main baseline artifact is available yet. This run establishes the initial measurement.

Provider	Metric	Main baseline	This PR	Impact	PR ceiling
Codex	Total thread wire	—	11.4 KiB	—	15.1 KiB	✅
Codex	Thread snapshot wire	—	5.7 KiB	—	7.3 KiB	✅
Codex	Live turn WebSocket wire	—	5.7 KiB	—	7.8 KiB	✅
Codex	Live turn WebSocket decoded	—	50.4 KiB	—	66.4 KiB	✅
Codex	Live turn messages	—	11	—	21	✅
Claude	Total thread wire	—	11.5 KiB	—	15.1 KiB	✅
Claude	Thread snapshot wire	—	5.7 KiB	—	7.3 KiB	✅
Claude	Live turn WebSocket wire	—	5.7 KiB	—	7.8 KiB	✅
Claude	Live turn WebSocket decoded	—	51.2 KiB	—	66.4 KiB	✅
Claude	Live turn messages	—	11	—	21	✅

Baseline: unavailable · PR result: 3b4637e · 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: 95.6 KiB
• Claude decoded thread snapshot: 96.3 KiB

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