Add deterministic overflow archive compaction

[chubes4]

Summary

• Add an overflow archive branch to conversation compaction that splits oversized ordered inputs deterministically before any summarizer call.
• Return archive items verbatim alongside retained messages with a synthetic archive stub and pointer metadata for consumer persistence.
• Extend smoke coverage for large input, no-op small input, single unsplittable input, and deterministic output.

Testing

• 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 the implementation and smoke tests; Chris remains responsible for review and merge.

Fixes #15