Add overmind signal command

[paddor]

Summary

• Adds a new overmind signal <SIGNAL> [process name...] command to send arbitrary signals to named processes (or all if none specified)
• Supports wildcard matching for process names, consistent with stop and restart
• Adds HUP to signalMap (was missing, primary use case for config reload)

Usage examples

overmind signal HUP web        # send SIGHUP to "web" process
overmind signal USR1 worker*   # send SIGUSR1 to processes matching "worker*"
overmind signal TERM           # send SIGTERM to all processes

Implementation

Follows the same patterns as stop and restart:

• cmd_signal.go — CLI handler, validates signal name arg is present, sends command over socket
• start/command_center.go — processSignal() parses signal name via signalMap, sends to matched processes
• start/process.go — Signal() method sends signal via groupSignal() without modifying lifecycle flags (pure signal passthrough)
• start/handler.go — Adds HUP to signalMap
• main.go — Registers the new command

Test plan

• go build compiles successfully
• overmind signal HUP <process> sends SIGHUP to a running process
• overmind signal USR1 web* sends to wildcard-matched processes
• overmind signal TERM (no process names) sends to all processes
• overmind signal INVALID is a silent no-op on the daemon side
• overmind signal (no args) prints a fatal error