Preserve literal preset env values for resume

[SiluPanda]

Summary

• preserve literal preset environment values in the private session manifest so a later resume gets the same configuration
• continue storing passthrough environment entries by name, so values inherited from the caller (such as HF_TOKEN) are not copied into the manifest
• cover both the serialized YAML and its round-trip through the resume configuration loader

The manifest already uses the session's atomic private writer, which creates files with mode 0600. This change records KEY=value only when the original preset configuration contained that literal value; a bare KEY remains a passthrough reference resolved from the environment on resume.

Fixes #4165.

Validation

$ uv run pytest -q src/tests/_internal/cli/services/presets/test_agent.py src/tests/_internal/cli/services/presets/test_create.py
117 passed, 2 skipped in 2.45s

$ uv run ruff format --check <changed Python files>
2 files already formatted

$ uv run ruff check <changed Python files>
All checks passed!

$ git diff --check
# no output

AI-assisted implementation; I reviewed the diff and ran the checks above locally.