feat: add Honcho session-write dedup and metadata stripping

[NewTurn2017]

Summary

• bootstrap a dedicated branch/PR for feat(honcho): add session-write dedup and platform metadata stripping #9404 so Honcho write dedup and metadata hygiene can proceed independently
• keep scope focused on session-write dedup and platform metadata stripping before Honcho persistence

Planned implementation

• add a persisted dedup cursor / equivalent write guard
• prevent replayed or restored sessions from duplicating Honcho writes
• strip platform-only metadata before Honcho storage
• add restart/retry coverage tests

Status

• Draft PR created to reserve branch/PR scope
• Implementation commits will be added sequentially

Refs #9404

[teknium1]

Thanks for reserving a focused scope. There is not yet an implementation to review: gh pr view 9422 reports zero changed files, and gh pr diff 9422 contains no patch.

Problems

• Current main still writes each locally unsynced item using only peer.message(msg["content"]) before add_messages() (plugins/memory/honcho/session.py:436-448); no source-event identity or persisted watermark is present.
• HonchoMemoryProvider.sync_turn() appends chunks from user/assistant content and flushes them (plugins/memory/honcho/__init__.py:1214-1239), so the proposed replay-safe behavior needs concrete wiring and coverage.

Suggested changes

• Add source-event idempotence rather than content-hash deduplication, so identical text in separate turns remains valid evidence.
• Include restart/retry/backfill tests once the implementation is added.

Automated hermes-sweeper review.