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/user/slim.go

@@ -0,0 +1,42 @@
+package user
+
+import (
+	gitea_sdk "code.gitea.io/sdk/gitea"
+)
+
+func slimUserDetail(u *gitea_sdk.User) map[string]any {
+	if u == nil {
+		return nil
+	}
+	return map[string]any{
+		"id":         u.ID,
+		"login":      u.UserName,
+		"full_name":  u.FullName,
+		"email":      u.Email,
+		"avatar_url": u.AvatarURL,
+		"html_url":   u.HTMLURL,
+		"is_admin":   u.IsAdmin,
+	}
+}
+
+func slimOrg(o *gitea_sdk.Organization) map[string]any {
+	if o == nil {
+		return nil
+	}
+	return map[string]any{
+		"id":          o.ID,
+		"name":        o.Name,
+		"full_name":   o.FullName,
+		"description": o.Description,
+		"avatar_url":  o.AvatarURL,
+		"website":     o.Website,
+	}
+}
+
+func slimOrgs(orgs []*gitea_sdk.Organization) []map[string]any {
+	out := make([]map[string]any, 0, len(orgs))
+	for _, o := range orgs {
+		out = append(out, slimOrg(o))
+	}
+	return out
+}