subagents: add availableToSubagents opt-in toggle for extraMcpServers

hive-subagent-mcp/src/paths.rs

@@ -1,4 +1,4 @@
-//! The one per-agent path this daemon needs.
+//! The per-agent paths this daemon needs.
 
 use std::path::PathBuf;
 
@@ -13,3 +13,28 @@ pub fn agent_socket() -> PathBuf {
         PathBuf::from,
     )
 }
+
+/// Durable state directory for the current agent. Same resolution as
+/// `hive-agent`'s own `paths::state_dir` — reads `HYPERHIVE_STATE_DIR`
+/// (always injected via `systemd.globalEnvironment` by the meta flake),
+/// falling back to the `HIVE_LABEL`-derived pattern for dev/test
+/// environments where the env var may not be set. Copied rather than shared
+/// with `hive-agent` (a binary crate with no lib target) — a five-line env
+/// read isn't worth a crate dependency to dedupe.
+#[must_use]
+pub fn state_dir() -> PathBuf {
+    if let Some(p) = std::env::var_os("HYPERHIVE_STATE_DIR") {
+        return PathBuf::from(p);
+    }
+    let label = std::env::var("HIVE_LABEL").unwrap_or_default();
+    PathBuf::from(format!("/agents/{label}/state"))
+}
+
+/// Harness-internal scratch dir this daemon writes its own generated
+/// subagent `--mcp-config` file into (see `crate::mcp_config`). Delegates to
+/// the shared resolver so this and `hive-agent`'s own harness dir always
+/// agree on the same path.
+#[must_use]
+pub fn harness_dir() -> PathBuf {
+    hive_agent_sock::paths::harness_dir()
+}