subagents: fold hive-extra-mcp into hive-agent-sock per mara's rescoping

CLAUDE.md

@@ -123,15 +123,18 @@ hand-maintained per-file tree drifts out of sync with the code.
   + manager socket (`/run/hive/mcp.sock`), the protocol an agent's harness
   speaks to `hive-c0re`. Same split rationale as the two above; the shared
   payload types it references stay in `hive-sh4re`.
-- **`hive-agent-sock/`** — wire types for the *in-agent* socket, served by
-  the harness to in-container producers — matrix / bash MCP daemons and
-  `forge_notify` are the built-in ones, but any user-configured MCP server
-  can push todos here too, nothing restricts the `subsystem` set. Carries
-  the loose-ends-v2 **todo** ops plus
-  harness-local reminders. ⚠️ Distinct from
+- **`hive-agent-sock/`** — what's common across the in-container "agent
+  plugins" (matrix/bash/subagent MCP daemons, `forge_notify`, any
+  user-configured MCP server). Wire types for the *in-agent* socket, served
+  by the harness to those producers — nothing restricts the `subsystem` set
+  on the todo ops, any producer can push its own. Carries the loose-ends-v2
+  **todo** ops plus harness-local reminders. ⚠️ Distinct from
   `hive-core-agent-sock` above: **this socket never leaves the container**
   and `hive-c0re` is not in the path at all — no broker round-trip, no
-  long-poll, no marker files.
+  long-poll, no marker files. Also carries `extra_mcp`: the
+  `hyperhive.extraMcpServers` spec + parsing, shared by `hive-agent` and
+  `hive-subagent-mcp` — not a socket protocol, just the natural home since
+  both already depend on this crate.
 - **`hive-types/`** — zero-dependency (bar serde) leaf crate holding the
   foundational newtypes, chiefly `Ident` (1–63 chars of `[a-z0-9-]`,
   constructed only via the validating parser). Lets every wire-type crate