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-ag3nt/assets/agent.css

@@ -171,6 +171,15 @@ pre.diff {
   font-size: 0.8em;
   letter-spacing: 0.05em;
 }
+.model-chip {
+  display: inline-block;
+  padding: 0.1em 0.6em;
+  border: 1px solid var(--purple-dim);
+  border-radius: 999px;
+  color: var(--cyan);
+  font-size: 0.78em;
+  letter-spacing: 0.04em;
+}
 .btn-dashlink {
   color: var(--cyan);
   border: 1px solid var(--cyan);