hive-subagent-mcp: name a role at dispatch, load it as the system prompt

A `start` may now name a role: `role: "reviewer"` loads the spawning
agent's own `subagent_roles/reviewer.md` and renders it, alone, into one
per-session file that `--append-system-prompt-file` points at. The role is
the system prompt; the task is the turn, never the other way round — a
task baked into the system prompt would re-assert itself as an
instruction on every later turn of a continued session, not just the one
it was written for. The task instructions (`prompt_file`) are read and
folded ahead of the turn's own prompt instead, the same channel that
carries them to the subagent without a role.

The argument is optional, so every existing call is unchanged — pinned by
a test that a pre-role payload still deserializes with `role` absent from
the schema's required set, and another that the no-role path reaches
claude with the caller's own file, unrendered, and the trigger untouched.
With a role, one test pins the system-prompt file to the role's text and
nothing of the task, and another pins the task still reaching the
subagent as the turn's prompt.

A role name with no file fails the call, before the session name is even
reserved, and the error lists the roles the directory does hold. No agent
ships roles yet, so named-but-missing is the ordinary first-run state; a
fallback there would spawn a subagent under a prompt missing every clause
the role existed to carry. An empty file and a name that is not a plain
identifier refuse the same way.

hive-subagent-mcp/src/lib.rs

@@ -19,4 +19,5 @@
 pub mod mcp;
 pub mod mcp_config;
 pub mod paths;
+pub mod role;
 pub mod session;