Persistent agent env drift can set AGENTD_AGENT_COMMAND=agent and break services

Observed during intent-coder investigation: persistent agent env files can contain AGENTD_AGENT_COMMAND=agent (bare command) while systemd PATH for agentd-agent service does not resolve that binary. Result is crash-loop with exit 127 and missing FIFO. intent-coder recovered after agentd start rewrote env to absolute binary path. sd-coder currently shows same failure in journal. Need harden env writing/migration so existing agents always get absolute AGENTD_AGENT_COMMAND and optionally self-heal on list/status/start.