Fix aarch64 cross-compile: switch reqwest to rustls-tls

[zackkitzmiller]

Problem

The release workflow cross-compiles to aarch64-unknown-linux-gnu via cross, and the build fails:

Could not find openssl via pkg-config
The system library `openssl` required by crate `openssl-sys` was not found.

reqwest's default default-tls feature uses native-tls → openssl-sys, which needs a system OpenSSL that isn't present in the cross build environment for the target arch.

Fix

Switch reqwest to rustls-tls (pure-Rust TLS, no system dependency). openssl-sys is now entirely absent from the dependency tree, so every target — including aarch64 — builds without a system OpenSSL.

Because this requires default-features = false, the other reqwest defaults (charset, http2, system-proxy) are re-listed explicitly so behavior is otherwise unchanged.

Verification

• cargo tree -i openssl-sys → no match (removed)
• cargo tree -i rustls → present via hyper-rustls/reqwest
• 277 tests pass, clippy clean (host x86_64)

Note: TLS to api.anthropic.com / the local proxy is unaffected — rustls handles standard HTTPS. Worth one cross-build of aarch64 on this PR to confirm the release matrix is green.