Fix/ulw session restore

[zbochuan]

Related Issue

Fixes #191

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update

Changes Made

• Added mode, skip_tool_approval, ulw_turns, ulw_turns_used to the session reconstruction whitelist in Agent.input()
• Uses the same whitelist pattern rather than dict(session), so transient keys (permissions, stop_signal, pending_tool) remain blocked
• Added test_input_preserves_plugin_session_fields and test_input_does_not_leak_transient_keys

Testing

• I have tested this code locally
• All existing tests pass
• I have added tests for new functionality

Checklist

• My code follows the project's code style
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• New and existing unit tests pass locally
• Any dependent changes have been merged and published

[sanmaxdev]

The explicit session-field allowlist preserves ULW state without restoring unrelated request data. The focused ULW, approval, and session tests pass, and the new regression coverage checks both preservation and exclusion.