AgentProviderSchema was z.enum() which caused old clients to reject
session messages containing unknown providers (pi, copilot), breaking
the entire session. Changed to z.string() for future clients.
For currently deployed clients (<0.1.45), the daemon now filters out
unknown providers based on the appVersion sent in the WebSocket hello
message. Clients that don't send appVersion only see claude/codex/opencode.
a17f7d2d20