feat: Phase 5 polish — conditional LSP tools, capabilities, sidecar hooks

- ToolSide enum: documented Sidecar future variant
- StartSession.capabilities: client reports LSP availability
- Client detects LSP binaries on PATH, sends ["lsp_rust", "lsp_typescript"]
- build_tool_definitions() conditionally registers LSP tools only when
  client has LSP capability — model won't hallucinate unavailable tools
- CodeSession stores capabilities, has_lsp(), has_capability() accessors
- git_branch() reads from git for breadcrumb scoping
- ToolRegistry.gitea_client() accessor for reindex endpoint

src/integration_test.rs

@@ -658,6 +658,7 @@ mod grpc_tests {
                 file_tree: vec![],
                 model: "mistral-medium-latest".into(),
                 client_tools: vec![],
+                capabilities: vec![],
             })),
         })
         .await