Fix systemd agent dispatch for single-repo specs

[udaychandra]

Summary

• create auto-branches at the single target repo root instead of appending the repo path twice
• make background agent launch fail loudly when the transient user systemd unit does not produce a PID
• let agent status recover the running PID from the user systemd unit if the PID file is missing

Verification

• mvn -pl sail-harness,sail-infra -am -Dtest=AgentSessionTest,DispatchCommandTest -Dsurefire.failIfNoSpecifiedTests=false test
• mvn clean verify