Fix OT sync corruption: match Overleaf ShareJS ack/echo handling

The server broadcasts otUpdateApplied (with ops) to ALL clients including
the sender. Our bridge was treating its own echoed ops as remote ops and
re-applying them, causing text duplication (e.g. "simulatorimulator").

Rewrite OT handling to match Overleaf's ShareJS _onMessage pattern:
- ACK = no ops OR meta.source matches our publicId (own echo)
- REMOTE = ops from a different source
- ACK path calls onAck() without re-applying ops
- OtClient silently drops duplicate acks in synchronized state
- OtClient drops stale remote ops (version < current)
- Remove pendingEchos counter in favor of meta.source detection

Also: refresh MCP comment contexts on new-comment/delete-thread events,
add Overleaf reference repo to .gitignore.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

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