feat: sunbeam reindex-code CLI verb + ReindexCode proto

Proto: ReindexCode RPC with org/repo/branch filters.
CLI: sunbeam reindex-code [--org studio] [--repo owner/name] [--endpoint ...]
Calls Sol's gRPC ReindexCode endpoint, prints indexed symbol count.

sunbeam-proto/proto/code.proto

@@ -5,6 +5,19 @@ package sunbeam.code.v1;
 // the `sunbeam code` TUI client and Sol's server-side agent loop.
 service CodeAgent {
   rpc Session(stream ClientMessage) returns (stream ServerMessage);
+  rpc ReindexCode(ReindexCodeRequest) returns (ReindexCodeResponse);
+}
+
+message ReindexCodeRequest {
+  string org = 1;           // optional: filter to an org (empty = all)
+  string repo = 2;          // optional: specific repo (empty = all)
+  string branch = 3;        // optional: specific branch (empty = default)
+}
+
+message ReindexCodeResponse {
+  uint32 repos_indexed = 1;
+  uint32 symbols_indexed = 2;
+  string error = 3;         // empty on success
 }
 
 // ── Client → Sol ───────────────────────────────────────────────