feat: slim tool responses (#141)

Reduce token usage by slimming tool responses. Instead of returning full Gitea SDK objects (with nested user/repo objects, avatars, permissions, etc.), each operation now has a colocated `slim.go` that extracts only the fields an LLM needs. List endpoints return even fewer fields than single-item endpoints.

Other changes:
- Add `params` helpers to DRY parameter extraction across 40+ handlers
- Remove `{"Result": ...}` wrapper for flatter responses
- Reduce default pageSize from 100 to 30

Fixes: https://gitea.com/gitea/gitea-mcp/issues/128

*Created by Claude on behalf of @silverwind*

Reviewed-on: https://gitea.com/gitea/gitea-mcp/pulls/141
Reviewed-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: silverwind <me@silverwind.io>
Co-committed-by: silverwind <me@silverwind.io>

operation/label/slim_test.go

@@ -0,0 +1,25 @@
+package label
+
+import (
+	"testing"
+
+	gitea_sdk "code.gitea.io/sdk/gitea"
+)
+
+func TestSlimLabel(t *testing.T) {
+	l := &gitea_sdk.Label{
+		ID:          1,
+		Name:        "bug",
+		Color:       "#d73a4a",
+		Description: "Something isn't working",
+		Exclusive:   false,
+	}
+
+	m := slimLabel(l)
+	if m["name"] != "bug" {
+		t.Errorf("expected name bug, got %v", m["name"])
+	}
+	if m["color"] != "#d73a4a" {
+		t.Errorf("expected color, got %v", m["color"])
+	}
+}