feat(sync): Phase 1 — continue-on-error + adopt-on-conflict in user config sync

Autonomous decisions:
- Hand-rolled InMemoryInstalledServerRepository in tests — no existing InstalledServerRepository fake in mcpmux-core; mirrors InMemorySettingsRepository pattern.
- Simulated per-server install failures for continue/all-fail tests — JSON entries are valid; repo-level failure hooks isolate loop error handling.

Signed-off-by: crimsonsunset <jsangio1@gmail.com>

Author: crimsonsunset