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/index.html

@@ -15,6 +15,7 @@
 
   <div id="state-row">
     <span id="state-badge" class="state-badge state-loading">… booting</span>
+    <span id="model-chip" class="model-chip" hidden></span>
     <span id="last-turn" class="last-turn" hidden></span>
     <button type="button" id="cancel-btn" class="btn-cancel-turn" hidden>■ cancel turn</button>
   </div>