Relates to #377

Summary

• Feishu channel stuck in "重启中..." (Restarting) due to an infinite config write → OpenClaw restart → WS reconnect → syncAll() → config write loop
• Root cause: OpenClawConfigWriter.write() unconditionally wrote the config file on every syncAll() call, triggering OpenClaw's file watcher even when content was unchanged
• Fix: keep last-written content in memory and skip the write when it matches, breaking the loop after one legitimate write

Test plan

• New test: writes config on first call
• New test: skips write when content unchanged (mtime stays same)
• New test: writes when content actually changes
• New test: writes again when content reverts to original
• New test: skips repeated identical calls (restart loop scenario)
• New test: separate writer instances don't share state
• Manual: connect a Feishu channel and verify it stabilizes to "已连接" without restart loop

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