Resolve all architecture open questions, add 13 ADRs, update specs

Resolved all 11 open questions based on project guidance:

Transport:
- OQ-01/OQ-07: ACME/Let's Encrypt with domain + IP paths (ADR-008)
- OQ-02: Default to n0 relay, --iroh-relay override (ADR-009)
- OQ-05: Transport chaining supported natively (ADR-010)

Client:
- OQ-06: Programmatic-first API, no ~/.ssh/config (ADR-011)

Server:
- OQ-04: Ed25519 + OpenSSH cert-authority, no password auth (ADR-012)
- OQ-08: fail2ban-friendly logging + built-in rate limiting (ADR-013)

TUN:
- OQ-03/OQ-09: Deferred entirely, recommend tun2proxy (ADR-014)
- tun-shim.md marked deprecated

NAPI:
- OQ-10: Expose both connect() and serve() (ADR-016)
- OQ-11: Use napi-rs for FFI bridge (ADR-015)

Additional ADRs created during review:
- ADR-006: No logging of tunnel destinations (was phantom reference)
- ADR-017: Stealth mode protocol multiplexing
- ADR-018: Control channel for pubsub over SSH

Fixed: ADR-002 status → Superseded, ADR-007 title typo,
WRAUTH_SERVER typo, ADR-005 stale wraith-tun refs,
undefined ACL feature removed from server.md,
--proxy semantic difference documented.

docs/architecture/decisions/018-control-channel-for-pubsub.md

@@ -0,0 +1,38 @@
+# ADR-018: Control Channel for PubSub over SSH
+
+## Status
+Accepted
+
+## Context
+The NAPI wrapper and pubsub integration need a way to use wraith's SSH channel as a data plane for event routing. When a `wraith connect` client opens an SSH session to a server, the `direct_tcpip` channel type is used to reach specific TCP targets (host:port).
+
+For the pubsub use case, the client needs a dedicated bidirectional stream to the server's event bus — not a TCP connection to a random host. There are several approaches:
+
+1. **Special destination**: Use `direct_tcpip` with a reserved destination (e.g., `wraith-control:0`) that the server recognizes and routes internally instead of connecting to a TCP target.
+2. **Port forwarding**: The server runs a pubsub hub on a specific port (e.g., 9736) and the client uses normal port forwarding (`-L 9736:hub:9736`).
+3. **Custom channel type**: Define a new SSH channel type beyond `direct_tcpip` and `forwarded_tcpip`.
+
+## Decision
+Use approach 1: a reserved `direct_tcpip` destination string. When the server receives a `channel_open_direct_tcpip` request for `wraith-control:0`:
+
+1. The `channel_open_direct_tcpip` handler detects the special target via string matching
+2. Instead of connecting to a TCP target, it bridges the channel to the local pubsub event bus
+3. `EventEnvelope` JSON flows bidirectionally over the SSH channel
+
+The destination string `wraith-control` is reserved. Regular TCP targets are hostnames or IP addresses, so there is no collision risk.
+
+Approach 2 (port forwarding to a specific port) is still supported as an alternative — the client can use `--forward 9736:localhost:9736` if the server runs a pubsub hub on that port. But the control channel approach is simpler and doesn't require a separate listening port.
+
+Approach 3 (custom channel type) was rejected because russh's `direct_tcpip` handler is well-understood and adding custom channel types requires modifying russh.
+
+## Consequences
+- **Positive**: Simple implementation — just string matching in the server's `channel_open_direct_tcpip` handler.
+- **Positive**: No separate port or service needs to run on the server. The control channel is built into wraith.
+- **Positive**: Compatible with the NAPI wrapper's single-duplex-stream model.
+- **Positive**: Port forwarding to a specific port is still available as an alternative.
+- **Negative**: The string `wraith-control` is a magic constant. It should be defined as a constant in the crate.
+- **Negative**: Regular TCP destinations accidentally matching `wraith-control` would be misrouted. Mitigated by reserving the entire `wraith-` prefix namespace.
+
+## References
+- [napi-and-pubsub.md](../napi-and-pubsub.md)
+- [server.md](../server.md)