subagents: add availableToSubagents opt-in toggle for extraMcpServers

hive-subagent-mcp/src/mcp_config.rs

@@ -0,0 +1,56 @@
+//! Builds a subagent's own filtered `--mcp-config`: only
+//! `hyperhive.extraMcpServers` entries with `availableToSubagents = true`
+//! (see `hive_extra_mcp::ExtraMcpServer::available_to_subagents`) ever reach
+//! a subagent's claude invocation. Everything else — the built-in hyperhive
+//! surface (todos/messaging), the auto-injected `bash` and `subagent`
+//! entries — stays unreachable by construction: none of those default to
+//! opted in, and the built-in surface isn't an `extraMcpServers` entry at
+//! all, so there's no name for an operator to opt it in under even if they
+//! wanted to.
+
+use std::path::PathBuf;
+
+/// Filename the rendered config lives at, under [`crate::paths::harness_dir`].
+const CONFIG_FILE: &str = "subagent-mcp-config.json";
+
+/// Render the subagent-eligible extra-MCP servers to a `--mcp-config` file,
+/// returning its path — or `None` when no entry opts in (or the render/write
+/// fails), so [`hive_claude::Config::mcp_config`] stays unset and the
+/// subagent gets literally zero MCP servers, the same default as before this
+/// toggle existed. Re-rendered on every call (cheap: a filter plus a small
+/// file write) rather than cached once at daemon startup, so a config change
+/// takes effect on this subagent's next `start`/`continue` without needing
+/// the daemon itself restarted.
+#[must_use]
+pub fn build() -> Option<PathBuf> {
+    let state_dir = crate::paths::state_dir();
+    let servers: serde_json::Map<String, serde_json::Value> = hive_extra_mcp::load_extra_mcp()
+        .into_iter()
+        .filter(|(_, spec)| spec.available_to_subagents())
+        .map(|(name, spec)| (name, spec.to_json_entry(&state_dir)))
+        .collect();
+    if servers.is_empty() {
+        return None;
+    }
+    let body = serde_json::to_string_pretty(&serde_json::json!({ "mcpServers": servers }))
+        .unwrap_or_else(|_| "{}".into());
+    let dir = crate::paths::harness_dir();
+    if let Err(e) = std::fs::create_dir_all(&dir) {
+        tracing::warn!(
+            error = ?e,
+            dir = %dir.display(),
+            "subagent mcp-config: harness dir create failed; subagent gets zero extra MCP servers this turn",
+        );
+        return None;
+    }
+    let path = dir.join(CONFIG_FILE);
+    if let Err(e) = std::fs::write(&path, body) {
+        tracing::warn!(
+            error = ?e,
+            path = %path.display(),
+            "subagent mcp-config: write failed; subagent gets zero extra MCP servers this turn",
+        );
+        return None;
+    }
+    Some(path)
+}