fix: macOS process detection for session discovery

[bbsngg]

Summary

• Replace pgrep -af . with ps -eo pid,args in _scan_processes_via_pgrep() for macOS compatibility
• Keep pgrep as fallback if ps is unavailable
• Fixes the root cause where macOS pgrep only outputs PIDs without command lines, making agent pattern matching impossible

Fixes #1

Root Cause

macOS pgrep -af . outputs only bare PIDs (e.g., 35024), while Linux outputs PID command_line (e.g., 35024 node /usr/bin/codex). The downstream _matches_agent() regex cannot match agent patterns against pure numbers.

Solution

ps -eo pid,args is POSIX-standard and outputs PID full_command on both macOS and Linux. This is used as the primary method, with pgrep -af . retained as a fallback.

Test plan

• Verified on macOS Darwin 25.3.0 (arm64) — dashboard now shows 31 detected sessions
• Verify no regression on Linux
• Verify Windows path is unaffected (early return for os.name == "nt")

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