Kill reqwest — minimal HTTP client on raw hyper + tokio-rustls

New src/agent/api/http.rs: ~240 lines, supports GET/POST, JSON/form
bodies, SSE streaming via chunk(), TLS via rustls. No tracing dep.

Removes reqwest from the main crate and telegram channel crate.
Cargo.lock drops ~900 lines of transitive dependencies.

tracing now only pulled in by tui-markdown.

Co-Authored-By: Proof of Concept <poc@bcachefs.org>

src/agent/api/openai.rs

@@ -5,9 +5,9 @@
 // Also used for local models (Qwen, llama) via compatible servers.
 
 use anyhow::Result;
-use reqwest::Client;
 use tokio::sync::mpsc;
 
+use super::http::HttpClient;
 use super::types::*;
 use super::StreamEvent;
 
@@ -15,7 +15,7 @@ use super::StreamEvent;
 /// parsed StreamEvents through the channel. The caller (runner)
 /// handles routing to the UI.
 pub(super) async fn stream_events(
-    client: &Client,
+    client: &HttpClient,
     base_url: &str,
     api_key: &str,
     model: &str,