fix(scheduler,remote): post-cancel user turns + attachment sync reopen

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Summary

Fixes two critical correctness bugs found in recent scheduler/remote attachment changes.

Bug 1: Post-cancel user messages silently dropped

• Impact: After cancel_session_tasks, if the user immediately sent a new message while the cancel drain flag was still set, dispatch skipped it with CancelledError — the session appeared dead until the flag cleared.
• Root cause: Skip logic keyed only on _cancelled_sessions membership, with no distinction between pre-cancel queued turns and post-cancel new user submits.
• Fix: Increment per-session cancel epoch on cancel; stamp enqueue_cancel_epoch on submit/inject; skip only when stamped epoch is older than current epoch (agent_wake/system inject semantics preserved).

Bug 2: Feishu attachment sync fails after worker reconnect

• Impact: Remote workspace state survived Core TTL, but after worker reconnect file_blob_write returned SESSION_NOT_OPEN; attachments were not mirrored to .macchiato/inbox while the turn continued in remote mode.
• Root cause: attachment_sync called file_blob_write directly without the reopen path used by bash_tool.
• Fix: Retry via open_workspace on session-not-open errors.

Validation

• tests/test_kernel_runtime.py (cancel epoch + post-cancel user turn)
• tests/test_attachment_sync.py::test_sync_reopens_worker_session_on_not_open