Lazy Reconnect for GreptimeDB
Previously, if GreptimeDB was unavailable at startup, devtap permanently fell back to file store until restarted.
Now devtap wraps the unavailable store in a lazy reconnect layer that retries on each drain/status call (30s cooldown). The MCP server starts immediately with file store and reconnects to GreptimeDB transparently once it recovers.
- Startup failure enters multi-source mode: file store + lazy GreptimeDB wrapper
- Cooldown errors include the original connection error for diagnostics
- Normal startup (GreptimeDB available) is unchanged