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>

channels/telegram/Cargo.toml

@@ -9,7 +9,6 @@ capnp-rpc = "0.25"
 dirs = "6"
 futures = "0.3"
 poc-memory = { path = "../.." }
-reqwest = { version = "0.13", default-features = false, features = ["json", "form", "rustls"] }
 serde = { version = "1", features = ["derive"] }
 serde_json = "1"
 tokio = { version = "1", features = ["full"] }