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/milestone/slim.go

@@ -0,0 +1,28 @@
+package milestone
+
+import (
+	gitea_sdk "code.gitea.io/sdk/gitea"
+)
+
+func slimMilestone(m *gitea_sdk.Milestone) map[string]any {
+	if m == nil {
+		return nil
+	}
+	return map[string]any{
+		"id":            m.ID,
+		"title":         m.Title,
+		"description":   m.Description,
+		"state":         m.State,
+		"open_issues":   m.OpenIssues,
+		"closed_issues": m.ClosedIssues,
+		"due_on":        m.Deadline,
+	}
+}
+
+func slimMilestones(milestones []*gitea_sdk.Milestone) []map[string]any {
+	out := make([]map[string]any, 0, len(milestones))
+	for _, m := range milestones {
+		out = append(out, slimMilestone(m))
+	}
+	return out
+}