fix(prompt): place scratchpad under session folder

[ngoclam9415]

Summary

• resolve scratchpad paths under each agent session folder
• invalidate cached codec system prompts when session id changes
• add regression coverage for session scratchpad layout and prompt rerendering

Tests

• uv run pytest tests/unit/core/star/test_set_session_id.py tests/unit/test_prompt_api_repository.py tests/unit/core/skills/test_dana_skills_prompt_context.py -q
• uv run ruff check dana/core/prompt/environment_info.py dana/core/runtime/codec/codec_base.py dana/core/agent/star_agent.py tests/unit/core/star/test_set_session_id.py
• uv run python -m compileall dana/core/prompt/environment_info.py dana/core/runtime/codec/codec_base.py dana/core/agent/star_agent.py tests/unit/core/star/test_set_session_id.py