feat(code): tree-sitter symbol extraction + auto-indexing

Symbol extraction (symbols.rs):
- tree-sitter parsers for Rust, TypeScript, Python
- Extracts: functions, structs, enums, traits, classes, interfaces
- Signatures, docstrings, line ranges for each symbol
- extract_project_symbols() walks project directory
- Skips hidden/vendor/target/node_modules, files >100KB

Proto: IndexSymbols + SymbolEntry messages for client→server symbol relay

Client: after SessionReady, extracts symbols and sends IndexSymbols
to Sol for indexing into the code search index.

14 unit tests for symbol extraction across Rust/TS/Python.

sunbeam-proto/proto/code.proto

@@ -16,9 +16,28 @@ message ClientMessage {
     ToolResult tool_result = 3;
     ToolApproval approval = 4;
     EndSession end = 5;
+    IndexSymbols index_symbols = 6;
   }
 }
 
+message IndexSymbols {
+  string project_name = 1;
+  string branch = 2;
+  repeated SymbolEntry symbols = 3;
+}
+
+message SymbolEntry {
+  string file_path = 1;
+  string name = 2;
+  string kind = 3;
+  string signature = 4;
+  string docstring = 5;
+  int32 start_line = 6;
+  int32 end_line = 7;
+  string language = 8;
+  string content = 9;
+}
+
 message StartSession {
   string project_path = 1;
   string prompt_md = 2;