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/user/slim.go

@@ -4,21 +4,6 @@ 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