forge v0.4.0 — background processes

Servers keep running while the agent tests them

• bash {command, background: true} — starts the process detached, returns immediately with its pid + a live log file, and keeps it running while the agent continues: curl it, run a client, tail the log, kill the pid.
• A trailing & triggers the same path automatically (&& chains do not).
• An instant crash — taken port, compile error — is caught within ~1s and reported with its real output instead of pretending to run.
• All background processes are cleaned up when the forge session exits.

Also includes the v0.3.1 pathless write_file fix.

Full changelog: v0.3.1...v0.4.0