Log Codex app-server compaction completion

Codex app-server compaction events updated counts and callbacks, but successful compactions were silent in Gateway logs unless user-facing notification policy changed. That made long-running Codex sessions harder to diagnose after context refreshes.

Added a runtime-gated Gateway log only for completed compaction events from the Codex app-server backend, while keeping chat and channel notices behind the existing notifyUser opt-in.

• Improves operator observability for long-running Codex sessions by making successful app-server context compactions visible in Gateway logs.
• Preserves existing user-channel behavior, avoiding new chat or Discord notifications for default-silent compaction events.
• Reduces debugging ambiguity by logging only the completed Codex app-server path instead of broadening compaction policy.

• Added a compaction runner regression for backend codex-app-server with completed=true.
• Verified default-silent compaction notices do not call onBlockReply unless notifyUser is enabled.
• PR validation covered focused Vitest compaction tests, Codex app-server projector tests, and diff check.