chore: prepare AgentCore 0.16.0 release

[zhanghanduo]

Summary

Release AgentCore 0.16.0, consuming the change fragments from #73.

• Changed: the invalid streamed tool-call retry now uses streaming; the attempt events, the accepted_degraded outcome and the error tool result are described for downstream migration. Adds the jsonschema>=4.20,<5 dependency.
• Added: LoopConfig.stream_transport, call_llm(stream=...) and LoopConfig.tool_argument_validation.

The migration note was written as a .breaking.md fragment in this PR and consumed by prepare_release.py, so the attempt-event changes appear under "Changed" for ApodexHarness and FrontierAgentInternal.

Verification

• scripts/check_version_bump.py: Release validated: 0.15.0 -> 0.16.0
• Ruff, Pyright and the full suite (2306 passed, 2 skipped) pass.
• uv build produces apodex_agent_core-0.16.0 wheel and sdist.

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