fix: add provider warmup to prevent cold-start timeout on first channel message

The first API request after daemon startup consistently timed out (120s)
when using channels (Telegram, Discord, etc.), requiring a retry before
succeeding. This happened because the reqwest HTTP client's connection
pool was cold — no TLS handshake, DNS resolution, or HTTP/2 negotiation
had occurred yet.

The fix adds a `warmup()` method to the Provider trait that establishes
the connection pool on startup by hitting a lightweight endpoint
(`/api/v1/auth/key` for OpenRouter). The channel server calls this
immediately after creating the provider, before entering the message
processing loop.

Tested on Raspberry Pi 5 (aarch64) with OpenRouter + DeepSeek v3.2 via
Telegram channel. Before: first message took 2-7 minutes (120s timeout +
retries). After: first message responds in <30s with no retries.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/channels/mod.rs

@@ -426,6 +426,13 @@ pub async fn start_channels(config: Config) -> Result<()> {
         config.api_key.as_deref(),
         &config.reliability,
     )?);
+
+    // Warm up the provider connection pool (TLS handshake, DNS, HTTP/2 setup)
+    // so the first real message doesn't hit a cold-start timeout.
+    if let Err(e) = provider.warmup().await {
+        tracing::warn!("Provider warmup failed (non-fatal): {e}");
+    }
+
     let model = config
         .default_model
         .clone()