Summary

• Fixes Hindsight auto-retain to send only newly buffered turns when the API supports update_mode="append".
• Preserves the legacy full-session snapshot behavior for older Hindsight APIs that do not support append mode.
• Adds regression coverage for both append and legacy paths.

Root cause

Hermes probes for Hindsight update_mode="append" support and then reuses the stable session document id. But sync_turn() still serialized and resent every turn accumulated in the process on every retain. On live Palmer, this produced giant repeated async retain payloads; the embedded daemon spent minutes splitting multi-million-token batches, stopped answering /health within the 2s daemon-manager timeout, and the next client startup misclassified the busy Hindsight listener on port 9712 as a "non-hindsight process".

Test Plan

• PYTHONPATH=. pytest tests/plugins/memory/test_hindsight_provider.py::TestSyncTurn::test_sync_turn_appends_only_new_turns_when_append_supported tests/plugins/memory/test_hindsight_provider.py::TestSyncTurn::test_sync_turn_accumulates_full_session_without_append_support -q
• PYTHONPATH=. pytest tests/plugins/memory/test_hindsight_provider.py -q
• PYTHONPATH=. pytest tests/plugins/memory -q

Notes

This PR avoids the biggest runaway payload source. It does not mutate local Palmer Hindsight config or kill/restart daemons; that was explicitly out of scope without approval.