Fix forward-compatible provider handling for old app clients

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.

packages/server/src/shared/messages.ts

@@ -2653,6 +2653,7 @@ export const WSHelloMessageSchema = z.object({
   clientId: z.string().min(1),
   clientType: z.enum(["mobile", "browser", "cli", "mcp"]),
   protocolVersion: z.number().int(),
+  appVersion: z.string().optional(),
   capabilities: z
     .object({
       voice: z.boolean().optional(),