chore(backport): absorb upstream reliability fixes (batch2)

[bitifirefly]

Summary

Backport batch2 of upstream reliability fixes with focused, low-risk adaptations.

Included changes

• Subagent: file tools now resolve relative paths against workspace (consistent with main agent tools).
• Session manager: persist exact session key in JSONL metadata to avoid lossy filename reconstruction; improve list fallback for legacy files.
• Agent loop: allow one retry when model returns interim text before issuing tool calls, improving compatibility with providers that stage tool usage.

Tests

• Added tests/test_subagent_workspace_paths.py
• Added tests/test_agent_loop_interim_retry.py
• Extended tests/test_session_manager_storage_scope.py

Validation

• Targeted regressions passed.
• Full suite passed locally: 382 passed, 5 warnings.