Support HTTP/2 (h2) ALPN negotiation in supervisor L7 proxy

Problem

The supervisor's L7 TLS proxy hardcodes ALPN to http/1.1 on both the client-facing acceptor and the upstream connector (crates/openshell-supervisor-network/src/l7/tls.rs):

server_config.alpn_protocols = vec![b"http/1.1".to_vec()];
// and
config.alpn_protocols = vec![b"http/1.1".to_vec()];

This means any sandbox application that uses gRPC, Connect-RPC, or any protocol requiring HTTP/2 will fail with:

SSL routines:ssl3_read_bytes:tlsv1 alert no application protocol

The only workaround today is tls: skip for those endpoints, which bypasses L7 inspection entirely.

Context

RFC 0009 explicitly scopes h2 out of v1:

HTTP/2 and HTTP/3. The proxy's TLS termination pins ALPN to http/1.1 today, so these are not introspected.

Meanwhile, the gateway TLS config (crates/openshell-server/src/tls.rs) already negotiates h2:

config.alpn_protocols.extend([b"h2".to_vec(), b"http/1.1".to_vec()]);

So the gateway supports it, but the supervisor proxy doesn't.

Use case

AI coding agents (Cursor, Windsurf, etc.) increasingly use gRPC / Connect-RPC for their API communication, which requires h2. Running these inside OpenShell sandboxes forces tls: skip on all their API endpoints, losing the security benefit of L7 inspection.

Possible approaches

1. Full h2 L7 inspection — parse HTTP/2 frames in the MITM path. Most complete but significant effort.
2. h2 tunnel without inspection — detect h2 negotiation, relay frames without parsing. Loses L7 hooks but still does TLS termination.
3. Auto tls: skip fallback — if upstream only offers h2 (no http/1.1 fallback), automatically treat the endpoint as tls: skip. Pragmatic, minimal code change, and makes the current manual workaround automatic.

Approach 3 would cover the immediate gap with minimal risk.

Start by reading crates/openshell-supervisor-network/src/l7/tls.rs and comparing its ALPN handling with crates/openshell-server/src/tls.rs. Review RFC 0009 and determine whether the implementation should inspect h2, tunnel it, or fall back to tls: skip. Done means the selected behavior supports h2 clients without the current no-application-protocol failure and preserves the intended L7 security semantics.