NativeAgentServer::connect used `prompt_store.await?` which turned any
PromptStore initialization failure (permission denied, disk issues,
etc.) into a hard connection error, putting the ConversationView into
LoadError state and preventing all new native agent threads from being
created.

Changed to `.log_err()` so the error is logged at ERROR level but thread
creation proceeds with `prompt_store: None`. `NativeAgent::new` already
accepts `Option<Entity<PromptStore>>` and handles `None` gracefully —
user custom prompts won't load but threads still work.

Added a regression test that serializes a stale thread ID, loads the
panel (triggering the "last active thread not found in database"
warning), then dispatches NewThread through the real NativeAgentServer
path and verifies it produces a connected thread.

Release Notes:

- Agent panel now handles filesystem errors more gracefully