Add conversation compaction contract

[chubes4]

Summary

• Adds a public AgentConversationCompaction runtime contract for declarative policy normalization, safe transcript boundary selection, lifecycle event payloads, and summarizer-supplied compaction.
• Adds WP_Agent capability/config fields for conversation compaction support and policy discovery.
• Documents the public compaction surface and adds smoke coverage for no-op, success, failure, and tool-call/tool-result boundary behavior.

Fixes #7

Tests

• php -l agents-api.php && php -l src/Registry/class-wp-agent.php && php -l src/Runtime/AgentConversationCompaction.php && php -l tests/bootstrap-smoke.php && php -l tests/conversation-compaction-smoke.php && php -l tests/no-product-imports-smoke.php
• composer test

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (GPT-5.5)
• Used for: Drafted the public compaction contract, documentation, and smoke tests; Chris remains responsible for review and validation.