xmemory: opt-in mirroring of memorization-sweep transcripts (text-only)

[pufit]

What

Lets xmemory receive all of a session's text — the same message windows the memU memorization sweep already indexes — behind a new opt-in flag:

xmemory:
  index_conversations: true   # default: false

Until now xmemory only ever saw individual memorize-tool facts; the sweep was memU-only by design. With the flag set, every window the sweep indexes into memU (periodic sweep, session close, scheduled memorize, session delete) is also mirrored to xmemory as a transcript.

How

• XmemoryBridge.memorize_conversation(session_id, messages) — flattens messages to text-only lines ([ts] role: content), mirroring the memU sweep's payload contract exactly: role + content only — thinking and tool blocks/results are never sent. Transcripts are split at message boundaries into ~64 KB chunks (oversized single messages are hard-split) and each chunk is enqueued via write_async with FAST extraction — transcripts are high-volume; the configured extraction_logic still governs memorize-tool writes.
• AgentEngine.schedule_xmemory_transcript() — fire-and-forget background task (reuses the _memorize_bg_tasks lifecycle) called from _memorize_session, _memorize_incremental, and the session-delete route right where memU gets the same window. A slow/failing xmemory never extends the global memorize lock or affects the memU pass.
• Best-effort by design: the sweep watermark belongs to memU — a window whose xmemory write fails is logged and not retried. A failed chunk abandons the remaining chunks instead of hammering a down service.
• XmemoryConfig.index_conversations — off by default; full transcripts leave the machine only when the operator explicitly enables it. The xmemory section already requires restart on config reload, so no config_reload changes needed.

Not changed

• memU sweep behavior, watermarks, and the memorize/memory_recall handlers (only a stale comment updated).
• The CLI conversation-backfill path stays memU-only (recovery tool, constructs its own bridge).

Tests

17 new tests in tests/test_xmemory_bridge.py: transcript flattening excludes thinking/blocks/empty messages; chunk headers, boundaries, and oversized-message hard-split; opt-in gating (config default, unavailable bridge, engine scheduler); FAST extraction override; stop-on-first-failure; engine sweep mirroring the exact memU window; task-failure isolation. Full suite: 3014 passed.

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