Simplify codebase (#195)

Net **-650 LOC** by removing duplication and dead noise. All tests pass.

### Duplication & helpers
- Extracted shared slim helpers (`UserLogin`, `UserLogins`, `LabelNames`, `BodyWithAttachments`, `UserDetail`, `Repo`/`Repos`, `Label`/`Labels`) into `pkg/slim`. Deleted the 4 copies that lived in `issue/`, `pull/`, `search/`, `repo/` (plus duplicate `slimUserDetail`/`slimRepo`/`slimLabels` across packages).
- Added `params.GetOptionalBoolPtr` and `params.GetOptionalStringPtr`. Replaced 18 awkward `new(localVar); if !ok { = nil }` patterns across `repo/`, `pull/`, `issue/`, `label/`, `milestone/`, `search/`.
- Extracted `pullRequestReviewerFn` for the 99%-identical `createPullRequestReviewerFn`/`deletePullRequestReviewerFn` pair.

### Dead code & noise
- Deleted **122** `log.Debugf("Called X")` narration lines (`zap.AddCaller` already records the caller) and pruned 19 unused `log` imports.
- Removed the unused `log.Logger` wrapper; the mcp-go server now uses `log.Default().Sugar()` directly (matches `util.Logger`).
- Deleted dead `s.DeleteTools("")` — confirmed no-op in mcp-go.
- Stripped WHAT-narration comments per project guidance.

### Correctness & consistency
- Fixed `log.Errorf(err.Error())` format-string bug in `pkg/to/to.go` — a `%` in the error would have been interpreted as a directive.
- Standardized `to.TextResult`/`to.ErrorResult` usage; `release.go`, `tag.go`, `branch.go` were bypassing the helpers in 9 sites (skipping the wrapper's debug/error logging).
- Made `params.GetString` reject empty strings; dropped 21 redundant `err != nil || x == ""` checks in `operation/actions/`.
- Replaced raw `args["org"].(string)` in `ListOrgReposFn` with `params.GetString` to match the rest of the codebase.

### Performance
- **Cached `*gitea.Client` by host+token via `sync.Map`** + shared `*http.Transport` via `sync.Once` for both SDK and raw REST paths. Eliminates the SDK's `/api/v1/version` preflight on every tool call and enables connection keep-alive across requests.
- Gated `to.TextResult` debug log behind `flag.Debug` to skip the `string(bytes)` allocation when debug is off.
- Hoisted `8192` and `60s` magic numbers in `pkg/gitea/rest.go` into named constants.

---
This PR was written with the help of Claude Opus 4.7

---------

Co-authored-by: silverwind <silv3rwind@gmail.com>
Reviewed-on: https://gitea.com/gitea/gitea-mcp/pulls/195
Reviewed-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: silverwind <me@silverwind.io>
Co-committed-by: silverwind <me@silverwind.io>

operation/issue/slim.go

@@ -1,63 +1,11 @@
 package issue
 
 import (
-	"fmt"
-	"strings"
+	"gitea.com/gitea/gitea-mcp/pkg/slim"
 
 	gitea_sdk "code.gitea.io/sdk/gitea"
 )
 
-func userLogin(u *gitea_sdk.User) string {
-	if u == nil {
-		return ""
-	}
-	return u.UserName
-}
-
-func userLogins(users []*gitea_sdk.User) []string {
-	if len(users) == 0 {
-		return nil
-	}
-	out := make([]string, 0, len(users))
-	for _, u := range users {
-		if u != nil {
-			out = append(out, u.UserName)
-		}
-	}
-	return out
-}
-
-func labelNames(labels []*gitea_sdk.Label) []string {
-	if len(labels) == 0 {
-		return nil
-	}
-	out := make([]string, 0, len(labels))
-	for _, l := range labels {
-		if l != nil {
-			out = append(out, l.Name)
-		}
-	}
-	return out
-}
-
-func bodyWithAttachments(body string, atts []*gitea_sdk.Attachment) string {
-	links := make([]string, 0, len(atts))
-	for _, a := range atts {
-		if a == nil || a.DownloadURL == "" {
-			continue
-		}
-		links = append(links, fmt.Sprintf("[%s](%s)", a.Name, a.DownloadURL))
-	}
-	if len(links) == 0 {
-		return body
-	}
-	joined := strings.Join(links, "\n")
-	if body == "" {
-		return joined
-	}
-	return body + "\n\n" + joined
-}
-
 func slimIssue(i *gitea_sdk.Issue) map[string]any {
 	if i == nil {
 		return nil
@@ -68,15 +16,15 @@ func slimIssue(i *gitea_sdk.Issue) map[string]any {
 		"body":       i.Body,
 		"state":      i.State,
 		"html_url":   i.HTMLURL,
-		"user":       userLogin(i.Poster),
-		"labels":     labelNames(i.Labels),
+		"user":       slim.UserLogin(i.Poster),
+		"labels":     slim.LabelNames(i.Labels),
 		"comments":   i.Comments,
 		"created_at": i.Created,
 		"updated_at": i.Updated,
 		"closed_at":  i.Closed,
 	}
 	if len(i.Assignees) > 0 {
-		m["assignees"] = userLogins(i.Assignees)
+		m["assignees"] = slim.UserLogins(i.Assignees)
 	}
 	if i.Milestone != nil {
 		m["milestone"] = map[string]any{
@@ -107,13 +55,13 @@ func slimIssues(issues []*gitea_sdk.Issue) []map[string]any {
 			"title":      i.Title,
 			"state":      i.State,
 			"html_url":   i.HTMLURL,
-			"user":       userLogin(i.Poster),
+			"user":       slim.UserLogin(i.Poster),
 			"comments":   i.Comments,
 			"created_at": i.Created,
 			"updated_at": i.Updated,
 		}
 		if len(i.Labels) > 0 {
-			m["labels"] = labelNames(i.Labels)
+			m["labels"] = slim.LabelNames(i.Labels)
 		}
 		if i.Ref != "" {
 			m["ref"] = i.Ref
@@ -133,26 +81,9 @@ func slimComment(c *gitea_sdk.Comment) map[string]any {
 	return map[string]any{
 		"id":         c.ID,
 		"body":       c.Body,
-		"user":       userLogin(c.Poster),
+		"user":       slim.UserLogin(c.Poster),
 		"html_url":   c.HTMLURL,
 		"created_at": c.Created,
 		"updated_at": c.Updated,
 	}
 }
-
-func slimLabels(labels []*gitea_sdk.Label) []map[string]any {
-	out := make([]map[string]any, 0, len(labels))
-	for _, l := range labels {
-		if l == nil {
-			continue
-		}
-		out = append(out, map[string]any{
-			"id":          l.ID,
-			"name":        l.Name,
-			"color":       l.Color,
-			"description": l.Description,
-			"exclusive":   l.Exclusive,
-		})
-	}
-	return out
-}