feat: add persistent context compaction

[Sam Schillace (ramparte)]

Summary

• branch overall: add explicit stored-history compaction while preserving ephemeral request-time compaction behavior
• branch overall: preserve tool-call/tool-result pair validity during persistent compaction
• Attempt-2 remediation: place compaction notices before conversation messages so they cannot split an assistant tool call from its following result
• Attempt-2 remediation: leave stored history unchanged and report no_reduction when explicit compaction does not shrink it
• the notice-reserve budget guard predates Attempt 2 and remains unchanged by this remediation

Tests

• focused: uv run pytest -q tests/test_persistent_compaction.py tests/test_progressive_compaction.py tests/test_tool_pair_compaction.py (20 passed)
• full: uv run pytest -q (43 passed)
• protected existing regression test identity verified against origin/main
• git diff --check origin/main...HEAD

No lockfile changes.