per-agent extra MCP servers via hyperhive.extraMcpServers

new NixOS option in harness-base.nix:
  hyperhive.extraMcpServers.<key> = {
    command = "/path/to/server";
    args = [ ... ];
    env = { KEY = "value"; };
    allowedTools = [ "send_message" "join_room" ];  # or ["*"]
  };

declared as attrsOf submodule so agents stack arbitrarily many.
the module writes the whole map as JSON to
/etc/hyperhive/extra-mcp.json at activation; the harness reads
that file in mcp::render_claude_config and merges each entry
into the rendered --mcp-config under its own mcpServers.<key>
block. allowed_mcp_tools(flavor) extends the --allowedTools
arg with mcp__<key>__<pattern> for every entry — "*" (the
default) becomes mcp__<key>__* so every tool from that server
is auto-approved, or pass a concrete list to tighten.

collision guard: an extra server keyed "hyperhive" is dropped
with a warn-log so user config can't shadow the built-in
surface. malformed JSON / missing file fall back to "no
extras" silently.

prompt note added: agents see "(some agents only) extra MCP
tools surfaced as mcp__<server>__<tool>" and learn they're
declared via agent.nix. retires the matching TODO under
Per-agent extension. matrix-chat agents + bitburner-agent
migration unblocked.

hive-ag3nt/src/mcp.rs

@@ -580,10 +580,24 @@ pub fn allowed_mcp_tools(flavor: Flavor) -> Vec<String> {
             "ask_operator",
         ],
     };
-    names
+    let mut out: Vec<String> = names
         .iter()
         .map(|t| format!("mcp__{SERVER_NAME}__{t}"))
-        .collect()
+        .collect();
+    // Extra MCP servers declared via `hyperhive.extraMcpServers` in
+    // the agent's NixOS config. Each entry maps its `allowedTools`
+    // pattern list to `mcp__<server>__<pattern>` so claude can call
+    // them without per-tool operator approval. `["*"]` (the default)
+    // expands to `mcp__<server>__*` — every tool from that server.
+    for (server, spec) in load_extra_mcp() {
+        if server == SERVER_NAME {
+            continue;
+        }
+        for pat in spec.allowed_tools {
+            out.push(format!("mcp__{server}__{pat}"));
+        }
+    }
+    out
 }
 
 /// Combined allow-list passed to `--allowedTools` (auto-approve) — covers
@@ -605,20 +619,78 @@ pub fn builtin_tools_arg() -> String {
     ALLOWED_BUILTIN_TOOLS.join(",")
 }
 
+/// Where the NixOS module writes the per-agent extra-MCP spec (see
+/// `nix/templates/harness-base.nix`). Each entry becomes an additional
+/// `mcpServers.<key>` block in the rendered claude config + a
+/// `mcp__<key>__<tool>` pattern in `--allowedTools`.
+const EXTRA_MCP_PATH: &str = "/etc/hyperhive/extra-mcp.json";
+
+#[derive(Debug, serde::Deserialize)]
+struct ExtraMcpServer {
+    command: String,
+    #[serde(default)]
+    args: Vec<String>,
+    #[serde(default)]
+    env: std::collections::BTreeMap<String, String>,
+    #[serde(default = "default_allowed_tools")]
+    #[serde(rename = "allowedTools")]
+    allowed_tools: Vec<String>,
+}
+
+fn default_allowed_tools() -> Vec<String> {
+    vec!["*".to_owned()]
+}
+
+/// Read + parse the extra-MCP spec. Returns an empty map on missing /
+/// unparsable file (the agent has none configured, or the file is
+/// malformed — both cases degrade to "no extra servers").
+fn load_extra_mcp() -> std::collections::BTreeMap<String, ExtraMcpServer> {
+    let Ok(raw) = std::fs::read_to_string(EXTRA_MCP_PATH) else {
+        return std::collections::BTreeMap::new();
+    };
+    serde_json::from_str(&raw).unwrap_or_else(|e| {
+        tracing::warn!(
+            path = EXTRA_MCP_PATH,
+            error = ?e,
+            "extra-mcp spec parse failed; ignoring",
+        );
+        std::collections::BTreeMap::new()
+    })
+}
+
 /// Render the MCP config blob claude reads from `--mcp-config <path>`.
 /// `agent_binary` is the path (or PATH-resolvable name) of the `hive-ag3nt`
 /// executable; `socket` is the hyperhive per-agent socket bind-mounted into
-/// the container (forwarded to the child as `--socket <path>`).
+/// the container (forwarded to the child as `--socket <path>`). Merges in
+/// any extra MCP servers declared via `hyperhive.extraMcpServers` in the
+/// agent's NixOS config.
 #[must_use]
 pub fn render_claude_config(agent_binary: &str, socket: &std::path::Path) -> String {
-    let config = serde_json::json!({
-        "mcpServers": {
-            SERVER_NAME: {
-                "command": agent_binary,
-                "args": ["--socket", socket.display().to_string(), "mcp"],
-                "env": {}
-            }
+    let mut servers = serde_json::Map::new();
+    servers.insert(
+        SERVER_NAME.to_owned(),
+        serde_json::json!({
+            "command": agent_binary,
+            "args": ["--socket", socket.display().to_string(), "mcp"],
+            "env": {}
+        }),
+    );
+    for (name, spec) in load_extra_mcp() {
+        if name == SERVER_NAME {
+            tracing::warn!(
+                "extra MCP server name `{SERVER_NAME}` collides with the built-in surface; ignoring",
+            );
+            continue;
         }
-    });
+        servers.insert(
+            name,
+            serde_json::json!({
+                "command": spec.command,
+                "args": spec.args,
+                "env": spec.env,
+            }),
+        );
+    }
+    let config = serde_json::json!({ "mcpServers": servers });
     serde_json::to_string_pretty(&config).unwrap_or_else(|_| "{}".into())
 }