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/repo/slim.go

@@ -1,56 +1,11 @@
 package repo
 
 import (
+	"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 slimRepo(r *gitea_sdk.Repository) map[string]any {
-	if r == nil {
-		return nil
-	}
-	m := map[string]any{
-		"id":                r.ID,
-		"full_name":         r.FullName,
-		"description":       r.Description,
-		"html_url":          r.HTMLURL,
-		"clone_url":         r.CloneURL,
-		"ssh_url":           r.SSHURL,
-		"default_branch":    r.DefaultBranch,
-		"private":           r.Private,
-		"fork":              r.Fork,
-		"archived":          r.Archived,
-		"language":          r.Language,
-		"stars_count":       r.Stars,
-		"forks_count":       r.Forks,
-		"open_issues_count": r.OpenIssues,
-		"open_pr_counter":   r.OpenPulls,
-		"created_at":        r.Created,
-		"updated_at":        r.Updated,
-	}
-	if r.Owner != nil {
-		m["owner"] = r.Owner.UserName
-	}
-	if len(r.Topics) > 0 {
-		m["topics"] = r.Topics
-	}
-	return m
-}
-
-func slimRepos(repos []*gitea_sdk.Repository) []map[string]any {
-	out := make([]map[string]any, 0, len(repos))
-	for _, r := range repos {
-		out = append(out, slimRepo(r))
-	}
-	return out
-}
-
 func slimBranch(b *gitea_sdk.Branch) map[string]any {
 	if b == nil {
 		return nil
@@ -144,7 +99,7 @@ func slimRelease(r *gitea_sdk.Release) map[string]any {
 		"draft":        r.IsDraft,
 		"prerelease":   r.IsPrerelease,
 		"html_url":     r.HTMLURL,
-		"author":       userLogin(r.Publisher),
+		"author":       slim.UserLogin(r.Publisher),
 		"created_at":   r.CreatedAt,
 		"published_at": r.PublishedAt,
 	}