subagents: add availableToSubagents opt-in toggle for extraMcpServers

hive-extra-mcp/Cargo.toml

@@ -0,0 +1,18 @@
+[package]
+name = "hive-extra-mcp"
+edition.workspace = true
+version.workspace = true
+
+[lints]
+workspace = true
+
+[dependencies]
+serde.workspace = true
+serde_json.workspace = true
+tracing.workspace = true
+
+# Shared spec for `hyperhive.extraMcpServers` — see src/lib.rs. Split out of
+# `hive-agent` (its only consumer until now) so `hive-subagent-mcp` can build
+# its own filtered subagent `--mcp-config` without depending on a binary
+# crate that has no lib target — same rationale as the `*-sock` crate splits
+# elsewhere in this workspace.