Publish complete final results with finalized trace references

[areshand]

Why

The production report dropped the first six lines of an 85-line, sub-6KB final answer because result publication always kept only the last 80 lines. It also emitted the public report before normalized agent events appeared, so the first transcript referenced only lifecycle logs.

What changes

• Preserve the complete final message whenever it is within the 6KB public bound; truncate by UTF-8 bytes only when it actually exceeds the bound.
• After stopping a session, wait up to two seconds for each latest agent attempt to publish events.jsonl.
• Re-checkpoint after that bounded finalization wait before generating and delivering the report.
• Add regression tests for the 85-line case, Unicode byte bounds, late trace readiness, and timeout behavior.

Validation

• Control-server test suite — 62 passed

Deployment

Source-only PR. It does not deploy or change trace storage.