fix(embed): pin nested uvx fallback to Python 3.13

[r266-tech]

Summary

• default UV_PYTHON to 3.13 when hindsight-embed directly launches the nested uvx hindsight-api fallback
• replace missing, empty, or whitespace-only values while preserving explicit caller overrides
• leave installed, development, and bundled API commands unchanged

Fixes #2901.

Tests

• pytest tests/test_profile_daemon_config.py::test_daemon_sets_uv_python_only_for_uvx_fallback -q (5 passed)
• HINDSIGHT_API_EMBEDDINGS_PROVIDER=tei HINDSIGHT_API_RERANKER_PROVIDER=tei pytest tests/test_profile_daemon_config.py -q (19 passed)
• remaining hindsight-embed suite in the sparse environment (149 passed, 3 deselected; the three dependency-sensitive launcher cases are included in the 19-test run above)
• Ruff check and format for the changed files
• ty check hindsight_embed
• python -m py_compile for the changed Python files