fix: fully detach manifest service starts so Daytona evals don't wedge (#676)

[ElegantLin]

Summary

ManifestEnvironment.provision() started declared services with {cmd} > {log} 2>&1 & — stdout/stderr redirected, but stdin left attached and no nohup.

On Daytona, exec runs as a session command that only reports an exit code once nothing holds the session's streams (the failure class root-caused in smolclaws#85). A backgrounded service that inherits the session's stdin/stdout/stderr keeps the call "running" until the hard timeout cap (#670) → a manifest-driven eval wedges (~1h) instead of starting.

Docker is unaffected — communicate() returns when the foreground shell exits — so the bug is invisible in local runs and CI, and only bites on Daytona.

Fix

Start services with full detachment:

nohup {cmd} </dev/null >/tmp/benchflow-env-<svc>.log 2>&1 &

• all three fds redirected — the missing </dev/null (stdin) is the operative change; the old form only covered stdout/stderr
• nohup to detach from the session's SIGHUP
• no disown — it is a bash/zsh builtin and the Daytona DinD shell is sh/dash
• per-service logs stay retrievable at /tmp/benchflow-env-<svc>.log

Mirrors the fix that resolved the identical wedge downstream.

Test plan

• test_provision_fully_detaches_service_fds — asserts nohup + all three fd redirections + backgrounded + no disown; verified to fail on the old > {log} 2>&1 & form (fail-then-pass). The pre-existing test only checked the command ended with &, which is exactly why the missing stdin redirect slipped through.
• tests/environment/ + manifest inbound suite: 75 passed
• ruff check + format, ty clean

Closes #676

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