feature: verified macOS install and launchd service lifecycle #944

Feature Description

The install path is the first thing anyone touches and the easiest place to lose a user. macOS
is its own code path — launchd, plist rendering, launchctl bootstrap — and it is verified ad
hoc today on whatever machine the author had.

Proposed Solution

• Fresh install via curl | sh on a machine that has never run Cortex, arm64 and amd64
• Upgrade over an older install: service restarted, config preserved
• agentop uninstall leaves no launchd registration and no modified agent settings;
  ~/.cortex stays unless --purge, which deletes it too
• Re-running install is idempotent
• Service survives logout/login and a crash
• agentop service status | start | stop | restart accurate in every state
• stop and restart leave no proxy holding the ports, so the next start binds first
  time. Since Fix: Make a local Cortex restart quick and clean #1320 a local install closes on SIGTERM instead of draining for 15s, and
  the supervisor kills a stuck proxy after 3s, inside launchd's 5s ExitTimeOut
• Plist rendering safe for $HOME and --config paths containing XML metacharacters

Additional Context

Part of the Cortex v0.9.0 release. Gates exit criterion 1 (install works cold). Automated by
the macOS release smoke test.