Make a provider switch clear its settings and show at once in the TUI

[thesiti92]

Found testing 0.1.9 in the TUI against Whiteboard's settings.

Changes

• Provider switch clears its settings. api_key, endpoint, model and provider_details are marked x-reset-by = "provider". config set clears them when the provider takes a new value, so a switch in the TUI or the CLI no longer sends the old vendor's key to the new vendor, or to the old vendor's endpoint. The prompt and general settings (thresholds, tests, timeouts) stay. Setting the same provider again changes nothing. The manifest check rejects an x-reset-by that names no other option.
• The TUI rereads settings after a change. Values that follow another setting, such as the model default, now update at once instead of after reopening the screen.

Verified

• Store test: switching Anthropic → Gemini keeps only the prompt and min_lines. Settings test for the reread. Both fail without their fix.
• cargo test passes. The TUI suite passes apart from the shutdown signal test, which also fails on main locally.
• In tmux: the patched TUI against this build switches Anthropic (key, proxy endpoint, Sonnet, custom prompt) to Gemini. Key, endpoint and model clear, Model shows gemini-3.8-flash, and the prompt and min_lines stay.

For Whiteboard's next diffr bump: Whiteboard's save skips writes equal to values it read before saving. Once diffr clears the endpoint on a provider change, a custom endpoint kept across a switch would be skipped and lost. Its writes need to compare against values reread after each write.