Add macOS and iTerm2 support for backlog orchestrator

[lossyrob]

Summary

• add a native macOS launcher for Copilot worker terminals
• support explicit iTerm2 selection (including iterm/terminal2 aliases) and a no-surprise Terminal.app auto default
• use iTerm2's nonblocking create-session-then-write-command AppleScript flow
• allow time for macOS's one-time GitHub Copilot.app → iTerm.app Automation consent
• port backlog-orchestrator lifecycle commands and worker prompts away from Windows-only assumptions
• retain the existing Windows Terminal/PowerShell launch path
• update plugin metadata to version 0.1.23

Validation

• exercised prompt-file, resume, empty/non-empty Copilot argument, and terminal-selection paths under stock macOS Bash 3.2
• ran real iTerm2 new-window and current-window-tab integration tests using an observable mock Copilot command; both returned immediately and received exact arguments
• ran a real Terminal.app auto integration test with exact prompt argument verification
• compiled both embedded AppleScript launchers
• validated skill definitions and plugin JSON metadata

macOS permissions

The first explicit iTerm2 launch can trigger macOS's one-time Automation prompt for GitHub Copilot.app to control iTerm.app. The launcher cannot suppress system consent; it waits up to ten minutes for the decision. Subsequent launches do not prompt after access is allowed in System Settings → Privacy & Security → Automation.

Breaking changes

None. Windows behavior remains supported. macOS auto now deliberately selects Terminal.app; choose --terminal iterm2 for iTerm2.

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