Add upgrade all chat command with drain check and convergence verify

- `upgrade all` sweeps every registered sled in sequence, streaming
  per-sled phase markers and reporting a summary.
- Binary source is now the gateway's own artifact store via
  host.docker.internal (was: unresolvable `gateway` hostname serving
  the gateway's macOS binary to Linux sleds — would have bricked them).
- Sleds with active claude processes are skipped, never killed.
- After reconnect, /api/version git_hash is compared against the
  published artifact's .hash sidecar; divergence is reported loudly.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_019fHdm92yjvguPi2LiXfLB9

server/src/chat/transport/matrix/project_rebuild.rs

@@ -343,7 +343,7 @@ async fn wait_for_drain(container_name: &str, timeout_secs: u64) -> Option<Strin
 ///
 /// Uses `docker exec <name> pgrep -f claude` — exits 0 with PID list when found,
 /// exits 1 when no matches (treated as 0 active processes).
-async fn count_active_claude_processes(container_name: &str) -> Result<usize, String> {
+pub(crate) async fn count_active_claude_processes(container_name: &str) -> Result<usize, String> {
     let out = tokio::process::Command::new("docker")
         .args(["exec", container_name, "pgrep", "-f", "claude"])
         .output()