Summary

• Adds self-healing plugin re-sync on server startup that detects missing plugin packages and reinstalls them from resolved.txt
• Fixes Plugins fix reverted? #465: plugins disappear after server restart when the venv is recreated (e.g., uv upgrade in desktop app)
• Adds unit tests for the new _is_package_installed() and ensure_plugins_installed() methods

How it works

On every server startup, before plugin discovery runs, ensure_plugins_installed() checks whether each plugin listed in plugins.txt is actually installed in the environment. If all are present, it returns immediately (fast no-op). If any are missing (e.g., due to venv recreation from a uv upgrade), it reinstalls them from resolved.txt, which skips already-installed packages quickly.

Test plan

• uv run pytest tests/test_plugin_manager.py::TestIsPackageInstalled tests/test_plugin_manager.py::TestEnsurePluginsInstalled — 13 tests pass
• uv run daydream-scope --no-browser starts without errors
• Install a plugin, then uv pip uninstall <plugin> (simulating venv loss), restart server — plugin is reinstalled from manifest
• Restart server with all plugins present — no reinstall activity in logs (fast path)

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