Harden process lifecycle handling

[sylvesterdamgaard]

What changed

• Wire global.process_start_timeout into manager startup paths.
• Add a manager helper that bounds supervisor startup without tying child process lifetime to a timeout context.
• Use the bounded start helper for initial startup, API/TUI starts, restarts, scaling from zero, and reload starts.
• Replace direct parent-process kills with process-group SIGKILL for shutdown timeouts, health-check restarts, and memory-limit restarts.
• Add coverage that NewManager honors configured ProcessStartTimeout.

Why

process_start_timeout existed in config but was not used. Forced restarts also only killed the parent process in several paths, despite children being started in their own process group.

Validation

• git diff --check
• go test ./internal/process could not be run locally because go is not installed in this this environment.