dimos daemon mode with --daemon flag (systemd integration)

[dimos-bot]

Implement daemon mode for dimos that:

• Starts as systemd process (or equivalent) with --daemon flag
• ONLY enters daemon mode if startup completes with no errors
• Similar behavior to Docker daemon
• If errors on startup, fail gracefully and stay in foreground mode

Context: Agent needs to start dimos in background, run tests, iterate on modules.

Success criteria:

• dimos run <blueprint> --daemon starts background process
• Only daemonizes if no startup errors
• Returns process ID/handle for later management

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Synced from DIM-663