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/pull/slim.go

@@ -1,63 +1,11 @@
 package pull
 
 import (
-	"fmt"
-	"strings"
+	"gitea.com/gitea/gitea-mcp/pkg/slim"
 
 	gitea_sdk "code.gitea.io/sdk/gitea"
 )
 
-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 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 repoRef(r *gitea_sdk.Repository) map[string]any {
 	if r == nil {
 		return nil
@@ -81,8 +29,8 @@ func slimPullRequest(pr *gitea_sdk.PullRequest) map[string]any {
 		"merged":     pr.HasMerged,
 		"mergeable":  pr.Mergeable,
 		"html_url":   pr.HTMLURL,
-		"user":       userLogin(pr.Poster),
-		"labels":     labelNames(pr.Labels),
+		"user":       slim.UserLogin(pr.Poster),
+		"labels":     slim.LabelNames(pr.Labels),
 		"comments":   pr.Comments,
 		"created_at": pr.Created,
 		"updated_at": pr.Updated,
@@ -91,7 +39,7 @@ func slimPullRequest(pr *gitea_sdk.PullRequest) map[string]any {
 	if pr.HasMerged {
 		m["merged_at"] = pr.Merged
 		m["merge_commit_sha"] = pr.MergedCommitID
-		m["merged_by"] = userLogin(pr.MergedBy)
+		m["merged_by"] = slim.UserLogin(pr.MergedBy)
 	}
 	if pr.Head != nil {
 		head := map[string]any{"ref": pr.Head.Ref, "sha": pr.Head.Sha}
@@ -117,7 +65,7 @@ func slimPullRequest(pr *gitea_sdk.PullRequest) map[string]any {
 		m["changed_files"] = *pr.ChangedFiles
 	}
 	if len(pr.Assignees) > 0 {
-		m["assignees"] = userLogins(pr.Assignees)
+		m["assignees"] = slim.UserLogins(pr.Assignees)
 	}
 	if pr.Milestone != nil {
 		m["milestone"] = pr.Milestone.Title
@@ -141,7 +89,7 @@ func slimPullRequests(prs []*gitea_sdk.PullRequest) []map[string]any {
 			"draft":      pr.Draft,
 			"merged":     pr.HasMerged,
 			"html_url":   pr.HTMLURL,
-			"user":       userLogin(pr.Poster),
+			"user":       slim.UserLogin(pr.Poster),
 			"created_at": pr.Created,
 			"updated_at": pr.Updated,
 		}
@@ -152,7 +100,7 @@ func slimPullRequests(prs []*gitea_sdk.PullRequest) []map[string]any {
 			m["base"] = pr.Base.Ref
 		}
 		if len(pr.Labels) > 0 {
-			m["labels"] = labelNames(pr.Labels)
+			m["labels"] = slim.LabelNames(pr.Labels)
 		}
 		out = append(out, m)
 	}
@@ -167,7 +115,7 @@ func slimReview(r *gitea_sdk.PullReview) map[string]any {
 		"id":             r.ID,
 		"state":          r.State,
 		"body":           r.Body,
-		"user":           userLogin(r.Reviewer),
+		"user":           slim.UserLogin(r.Reviewer),
 		"comments_count": r.CodeCommentsCount,
 		"submitted_at":   r.Submitted,
 		"html_url":       r.HTMLURL,
@@ -196,7 +144,7 @@ func slimReviewComment(c *gitea_sdk.PullReviewComment) map[string]any {
 		"position":     c.LineNum,
 		"old_position": c.OldLineNum,
 		"diff_hunk":    c.DiffHunk,
-		"user":         userLogin(c.Reviewer),
+		"user":         slim.UserLogin(c.Reviewer),
 		"html_url":     c.HTMLURL,
 		"created_at":   c.Created,
 		"updated_at":   c.Updated,