fix(reflect): unwrap JSON answer envelopes

[xmh1011]

Summary

• unwrap reflect responses when a model returns the done-tool JSON envelope as visible answer text
• keep normal JSON answers intact by requiring reflect envelope keys and tightening trailing id cleanup
• strengthen the reflect tool prompt against JSON-envelope answers

Closes #2298

Tests

• uv run --no-sync pytest -q tests/test_reflect_agent.py::TestCleanAnswerText tests/test_reflect_agent.py::TestCleanDoneAnswer tests/test_reflect_prompt_builder.py
• uv run --no-sync ruff check hindsight_api/engine/reflect/agent.py hindsight_api/engine/reflect/prompts.py tests/test_reflect_agent.py tests/test_reflect_prompt_builder.py
• uv run --no-sync ruff format --check hindsight_api/engine/reflect/agent.py hindsight_api/engine/reflect/prompts.py tests/test_reflect_agent.py tests/test_reflect_prompt_builder.py
• pre-commit hooks completed during git commit