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/pull/slim_test.go

@@ -0,0 +1,124 @@
+package pull
+
+import (
+	"testing"
+	"time"
+
+	gitea_sdk "code.gitea.io/sdk/gitea"
+)
+
+func TestSlimPullRequest(t *testing.T) {
+	now := time.Now()
+	additions := 10
+	deletions := 5
+	changedFiles := 3
+	pr := &gitea_sdk.PullRequest{
+		Index:     1,
+		Title:     "Fix bug",
+		Body:      "Fixes #123",
+		State:     "open",
+		Draft:     false,
+		HasMerged: false,
+		Mergeable: true,
+		HTMLURL:   "https://gitea.com/org/repo/pulls/1",
+		Poster:    &gitea_sdk.User{UserName: "bob"},
+		Labels: []*gitea_sdk.Label{
+			{Name: "bug"},
+			{Name: "priority"},
+		},
+		Comments:     2,
+		Created:      &now,
+		Updated:      &now,
+		Additions:    &additions,
+		Deletions:    &deletions,
+		ChangedFiles: &changedFiles,
+		Head: &gitea_sdk.PRBranchInfo{
+			Ref: "fix-branch",
+			Sha: "abc123",
+		},
+		Base: &gitea_sdk.PRBranchInfo{
+			Ref: "main",
+			Sha: "def456",
+		},
+		Assignees: []*gitea_sdk.User{
+			{UserName: "alice"},
+		},
+		Milestone: &gitea_sdk.Milestone{Title: "v1.0"},
+	}
+
+	m := slimPullRequest(pr)
+
+	if m["number"] != int64(1) {
+		t.Errorf("expected number 1, got %v", m["number"])
+	}
+	if m["title"] != "Fix bug" {
+		t.Errorf("expected title Fix bug, got %v", m["title"])
+	}
+	if m["user"] != "bob" {
+		t.Errorf("expected user bob, got %v", m["user"])
+	}
+	if m["additions"] != 10 {
+		t.Errorf("expected additions 10, got %v", m["additions"])
+	}
+	if m["milestone"] != "v1.0" {
+		t.Errorf("expected milestone v1.0, got %v", m["milestone"])
+	}
+
+	labels := m["labels"].([]string)
+	if len(labels) != 2 || labels[0] != "bug" {
+		t.Errorf("expected labels [bug priority], got %v", labels)
+	}
+
+	head := m["head"].(map[string]any)
+	if head["ref"] != "fix-branch" {
+		t.Errorf("expected head ref fix-branch, got %v", head["ref"])
+	}
+
+	assignees := m["assignees"].([]string)
+	if len(assignees) != 1 || assignees[0] != "alice" {
+		t.Errorf("expected assignees [alice], got %v", assignees)
+	}
+
+	// merged fields should not be present for unmerged PR
+	if _, ok := m["merged_at"]; ok {
+		t.Error("merged_at should not be present for unmerged PR")
+	}
+}
+
+func TestSlimPullRequests_ListIsSlimmer(t *testing.T) {
+	pr := &gitea_sdk.PullRequest{
+		Index:   1,
+		Title:   "PR title",
+		State:   "open",
+		HTMLURL: "https://gitea.com/org/repo/pulls/1",
+		Poster:  &gitea_sdk.User{UserName: "bob"},
+		Body:    "Full body text here",
+		Head:    &gitea_sdk.PRBranchInfo{Ref: "feature"},
+		Base:    &gitea_sdk.PRBranchInfo{Ref: "main"},
+	}
+
+	single := slimPullRequest(pr)
+	list := slimPullRequests([]*gitea_sdk.PullRequest{pr})
+
+	// Single has body, list does not
+	if _, ok := single["body"]; !ok {
+		t.Error("single PR should have body")
+	}
+	if _, ok := list[0]["body"]; ok {
+		t.Error("list PR should not have body")
+	}
+
+	// List has head as string ref, single has head as map
+	if _, ok := single["head"].(map[string]any); !ok {
+		t.Error("single PR head should be a map")
+	}
+	if list[0]["head"] != "feature" {
+		t.Errorf("list PR head should be string ref, got %v", list[0]["head"])
+	}
+}
+
+func TestSlimPullRequests_Nil(t *testing.T) {
+	if r := slimPullRequests(nil); len(r) != 0 {
+		t.Errorf("expected empty slice, got %v", r)
+	}
+}