fix: expand tilde in workspace path for claude-cli provider

cfg.Agents.Defaults.Workspace returns the raw config value (e.g.
~/.picoclaw/workspace) without tilde expansion. Use cfg.WorkspacePath()
which correctly expands ~ to the user's home directory, fixing a
"chdir: no such file or directory" error when using the claude-cli
provider.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

pkg/providers/http_provider.go

@@ -299,7 +299,7 @@ func CreateProvider(cfg *config.Config) (LLMProvider, error) {
 				}
 			}
 		case "claude-cli", "claudecode", "claude-code":
-			workspace := cfg.Agents.Defaults.Workspace
+			workspace := cfg.WorkspacePath()
 			if workspace == "" {
 				workspace = "."
 			}