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>

pkg/gitea/gitea.go

@@ -6,6 +6,7 @@ import (
 	"errors"
 	"fmt"
 	"net/http"
+	"sync"
 
 	mcpContext "gitea.com/gitea/gitea-mcp/pkg/context"
 	"gitea.com/gitea/gitea-mcp/pkg/flag"
@@ -13,21 +14,39 @@ import (
 	"code.gitea.io/sdk/gitea"
 )
 
+var (
+	clientCache     sync.Map // token -> *gitea.Client
+	sharedTransOnce sync.Once
+	sharedTrans     *http.Transport
+)
+
+func sharedTransport() *http.Transport {
+	sharedTransOnce.Do(func() {
+		sharedTrans = http.DefaultTransport.(*http.Transport).Clone()
+		if flag.Insecure {
+			sharedTrans.TLSClientConfig = &tls.Config{InsecureSkipVerify: true} //nolint:gosec // user-requested insecure mode
+		}
+	})
+	return sharedTrans
+}
+
+// NewClient returns a cached *gitea.Client keyed by host+token. The SDK's per-client
+// version cache and the shared transport let us reuse keep-alive connections
+// and avoid the SDK's /api/v1/version preflight on every tool call.
 func NewClient(token string) (*gitea.Client, error) {
-	httpClient := &http.Client{
-		Transport:     http.DefaultTransport,
-		CheckRedirect: checkRedirect,
+	key := flag.Host + "\x00" + token
+	if v, ok := clientCache.Load(key); ok {
+		return v.(*gitea.Client), nil
 	}
 
+	httpClient := &http.Client{
+		Transport:     sharedTransport(),
+		CheckRedirect: checkRedirect,
+	}
 	opts := []gitea.ClientOption{
 		gitea.SetToken(token),
+		gitea.SetHTTPClient(httpClient),
 	}
-	if flag.Insecure {
-		httpClient.Transport.(*http.Transport).TLSClientConfig = &tls.Config{
-			InsecureSkipVerify: true,
-		}
-	}
-	opts = append(opts, gitea.SetHTTPClient(httpClient))
 	if flag.Debug {
 		opts = append(opts, gitea.SetDebugMode())
 	}
@@ -35,10 +54,10 @@ func NewClient(token string) (*gitea.Client, error) {
 	if err != nil {
 		return nil, fmt.Errorf("create gitea client err: %w", err)
 	}
-
-	// Set user agent for the client
 	client.SetUserAgent("gitea-mcp-server/" + flag.Version)
-	return client, nil
+
+	actual, _ := clientCache.LoadOrStore(key, client)
+	return actual.(*gitea.Client), nil
 }
 
 // checkRedirect prevents Go from silently changing mutating requests (POST, PATCH, etc.)