model: runtime override via /model slash; fixes for port + bind

- runtime model override: Bus::{model,set_model} + POST /api/model
  (form-encoded {model: name}). turn.rs reads bus.model() per turn
  so a flip lands on the next claude invocation. /api/state grows
  a model field; agent page shows a 'model · <name>' chip in the
  state row. '/model <name>' slash command POSTs to the endpoint
  and refreshes state.

- port regression fix: agent_web_port no longer probes forward for
  *existing* agents (the previous fix shifted ports for any agent
  without a port file, including legacy ones whose container was
  already bound to the bare hashed port — dashboard rendered the
  new port, container was still on the old one, conn errors). new
  rule: port file exists → use it; absent + applied flake present
  → legacy, persist port_hash without probing; absent + no applied
  flake → fresh spawn, probe forward.

- SO_REUSEADDR on both the dashboard and per-agent web UI binds
  via tokio::net::TcpSocket. operator hit 12 retries failing on
  manager :8000 — REUSEADDR handles the TIME_WAIT case cleanly
  without a new dep; retry still covers the genuine
  process-still-alive overlap.

todo: drops the model-override entry (shipped); adds two new
items — model persistence (optional, future), and custom
per-agent MCP tools (groundwork for moving bitburner-agent into
hyperhive).

hive-c0re/src/dashboard.rs

@@ -72,13 +72,13 @@ pub async fn serve(port: u16, coord: Arc<Coordinator>) -> Result<()> {
 // `/messages/stream` for broker traffic.
 // ---------------------------------------------------------------------------
 
-/// Retry-on-AddrInUse bind. Same shape as the per-agent variant —
+/// `SO_REUSEADDR` bind with retry. Mirrors the per-agent variant —
 /// hive-c0re restarts also race the previous process's socket release.
 async fn bind_with_retry(addr: SocketAddr) -> Result<tokio::net::TcpListener> {
     let mut delay_ms = 250u64;
     let mut attempts = 0u32;
     loop {
-        match tokio::net::TcpListener::bind(addr).await {
+        match try_bind(addr) {
             Ok(l) => return Ok(l),
             Err(e) if e.kind() == std::io::ErrorKind::AddrInUse && attempts < 12 => {
                 tracing::warn!(
@@ -96,6 +96,16 @@ async fn bind_with_retry(addr: SocketAddr) -> Result<tokio::net::TcpListener> {
     }
 }
 
+fn try_bind(addr: SocketAddr) -> std::io::Result<tokio::net::TcpListener> {
+    let sock = match addr {
+        SocketAddr::V4(_) => tokio::net::TcpSocket::new_v4()?,
+        SocketAddr::V6(_) => tokio::net::TcpSocket::new_v6()?,
+    };
+    sock.set_reuseaddr(true)?;
+    sock.bind(addr)?;
+    sock.listen(1024)
+}
+
 async fn serve_index() -> impl IntoResponse {
     Html(include_str!("../assets/index.html"))
 }