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/issue/slim_test.go

@@ -0,0 +1,69 @@
+package issue
+
+import (
+	"testing"
+
+	gitea_sdk "code.gitea.io/sdk/gitea"
+)
+
+func TestSlimIssue(t *testing.T) {
+	i := &gitea_sdk.Issue{
+		Index:   42,
+		Title:   "Bug report",
+		Body:    "Something is broken",
+		State:   "open",
+		HTMLURL: "https://gitea.com/org/repo/issues/42",
+		Poster:  &gitea_sdk.User{UserName: "alice"},
+		Labels:  []*gitea_sdk.Label{{Name: "bug"}},
+		Milestone: &gitea_sdk.Milestone{
+			ID:    1,
+			Title: "v1.0",
+		},
+		PullRequest: &gitea_sdk.PullRequestMeta{HasMerged: false},
+	}
+
+	m := slimIssue(i)
+
+	if m["number"] != int64(42) {
+		t.Errorf("expected number 42, got %v", m["number"])
+	}
+	if m["body"] != "Something is broken" {
+		t.Errorf("expected body, got %v", m["body"])
+	}
+	if m["is_pull"] != true {
+		t.Error("expected is_pull true for issue with PullRequest")
+	}
+
+	ms := m["milestone"].(map[string]any)
+	if ms["title"] != "v1.0" {
+		t.Errorf("expected milestone title v1.0, got %v", ms["title"])
+	}
+}
+
+func TestSlimIssues_ListIsSlimmer(t *testing.T) {
+	i := &gitea_sdk.Issue{
+		Index:  1,
+		Title:  "Issue",
+		State:  "open",
+		Body:   "Full body",
+		Poster: &gitea_sdk.User{UserName: "alice"},
+		Labels: []*gitea_sdk.Label{{Name: "enhancement"}},
+	}
+
+	single := slimIssue(i)
+	list := slimIssues([]*gitea_sdk.Issue{i})
+
+	// Single has body, list does not
+	if _, ok := single["body"]; !ok {
+		t.Error("single issue should have body")
+	}
+	if _, ok := list[0]["body"]; ok {
+		t.Error("list issue should not have body")
+	}
+}
+
+func TestSlimIssues_Nil(t *testing.T) {
+	if r := slimIssues(nil); len(r) != 0 {
+		t.Errorf("expected empty slice, got %v", r)
+	}
+}