fix: warn at boot when the event loop cannot spawn subprocesses

[mickyarun]

What this does

On native Windows, running the backend with uvicorn --reload (or --workers) forces a SelectorEventLoop, which cannot spawn subprocesses. Every git call, repository scan, and CLI agent run then raises NotImplementedError from deep inside a request — surfacing as a 500 that the setup wizard blames on the path the user typed. A Windows evaluator was sent hunting for a missing .git in a checkout that had one.

This detects the condition once at startup and logs a loud, actionable remedy (run python dev_server.py, or drop --reload) while the cause is still attributable — instead of letting it resurface later as a mislabelled input error.

Notes

• The detector names ProactorEventLoop through getattr because that class exists only on Windows — dotted access threw AttributeError on macOS/Linux, i.e. on exactly the platforms the guard must stay silent on. That was a real portability bug in the original draft; it's fixed and covered.
• CLAUDE.md now documents python dev_server.py as the run command, with the --reload-on-Windows caveat spelled out.

Two adjacent fixes, same theme — a symptom pointing away from its cause

• Setup wizard add-path errors. It appended a "point to a git checkout" hint to every failure, including 5xx server errors unrelated to the path. It now shows that hint only when the backend actually rejected the path, and says "the server errored" otherwise.
• Zero-feature synthesis error. The message hard-coded "Claude" and asserted an MCP-registration failure it never verified. On an org using a local provider both are wrong — a different agent ran, and it registers no MCP server at all. It now names the provider that actually ran and quotes the agent's own final words.

Gates

mypy --strict, ruff, vue-tsc clean; 2545 backend tests pass, including new coverage for the event-loop detector (all branches) and the provider-named synthesis message.

[mickyarun]

Closing — the Windows NotImplementedError this addresses is already resolved for the client via run-command guidance (use python dev_server.py, not uvicorn --reload). The boot-time guard here is preventive, not a fix for a live issue, and it bundled two uncomplained-about changes. Dropping to keep scope tight; the branch is preserved locally if we want the guard later.