Release 0.9.5 - Fix context compression via sub-agent

Context compression now works correctly:
- Sub-agent receives prompt via shell pipeline (printf | codemax --sub-agent)
- Parses Summary: prefix from stop tool output
- Falls back to KEY EVENTS: pattern

Also includes:
- SystemPromptBuilder class (not yet integrated)
- docs/Modes_Flags.md
- tests/test_messages.cpp

All 16 tests pass.