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/lifecycle.rs

@@ -46,13 +46,18 @@ const DEFAULT_MEMORY_MAX: &str = "2G";
 const DEFAULT_CPU_QUOTA: &str = "50%";
 
 /// Returns the per-agent web UI port. Manager is fixed at `MANAGER_PORT`.
-/// For sub-agents the port is sticky once chosen: looked up from
-/// `agent_state_root(name)/port` if present, otherwise derived from
-/// the FNV-1a hash of the name and *probed forward* through the
-/// allocated range to skip any port another sub-agent has already
-/// claimed (birthday-paradox collisions are real even at 2–3
-/// agents). The chosen port is written back so subsequent calls
-/// resolve to the same value without re-probing.
+/// For sub-agents the port is sticky once chosen:
+///
+/// - **Port file present** (`state_root/port`): use it. End of story.
+/// - **Port file absent, applied flake present**: this is a legacy
+///   agent whose container is already bound to the bare
+///   `port_hash(name)`. Don't probe; just migrate by writing that
+///   value to the port file. The container stays where it is and
+///   subsequent renders agree with it.
+/// - **Port file absent, no applied flake**: this is a fresh spawn.
+///   Probe forward from `port_hash(name)` to skip any port another
+///   sub-agent has already claimed (via port file or legacy hash).
+///   Write the chosen port back.
 #[must_use]
 pub fn agent_web_port(name: &str) -> u16 {
     if name == MANAGER_NAME {
@@ -66,27 +71,36 @@ pub fn agent_web_port(name: &str) -> u16 {
     {
         return port;
     }
-    let taken = scan_taken_ports(name);
-    let start = port_hash(name);
-    let mut port = start;
-    for _ in 0..WEB_PORT_RANGE {
-        if !taken.contains(&port) {
-            break;
+    let applied_exists = crate::coordinator::Coordinator::agent_applied_dir(name).exists();
+    let chosen = if applied_exists {
+        // Legacy agent — container already running on the hashed
+        // port. Don't move it; just persist the value so future
+        // calls bypass this path.
+        port_hash(name)
+    } else {
+        let taken = scan_taken_ports(name);
+        let start = port_hash(name);
+        let mut port = start;
+        for _ in 0..WEB_PORT_RANGE {
+            if !taken.contains(&port) {
+                break;
+            }
+            port = next_port(port);
+            if port == start {
+                // Range fully exhausted (very unlikely — 900 slots) —
+                // give up and use the hashed value; collisions are
+                // surfaced as bind errors by the harness retry loop.
+                tracing::warn!(%name, "agent_web_port: range exhausted, returning hash");
+                break;
+            }
         }
-        port = next_port(port);
-        if port == start {
-            // Range fully exhausted (very unlikely — 900 slots) —
-            // give up and just use the hashed value; collisions are
-            // surfaced as bind errors by the harness retry loop.
-            tracing::warn!(%name, "agent_web_port: range exhausted, returning hash");
-            return start;
-        }
-    }
+        port
+    };
     let _ = std::fs::create_dir_all(&state_root);
-    if let Err(e) = std::fs::write(&port_file, format!("{port}\n")) {
+    if let Err(e) = std::fs::write(&port_file, format!("{chosen}\n")) {
         tracing::warn!(error = ?e, file = %port_file.display(), "persisting agent port failed");
     }
-    port
+    chosen
 }
 
 fn port_hash(name: &str) -> u16 {