Add compaction conservation metadata

[chubes4]

Summary

• Add generic compaction provenance metadata for original, compacted, retained, and archived item/byte counts.
• Add opt-in conservation checks that fail closed and return the original transcript unchanged when compaction drops too much durable content.
• Extend the compaction smoke test to cover healthy conservation, lossy rejection, disabled conservation, and metadata shape.

Closes #18.

Tests

• composer test
• php -l src/Runtime/AgentConversationCompaction.php
• php -l tests/conversation-compaction-smoke.php

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (GPT-5.5)
• Used for: Drafted implementation and tests; Chris reviewed via test/PR workflow.