debug: structured tool/HTTP logging, request IDs, config dump

Closes #13.

Today GITEA_DEBUG=true only flips the zap level to debug, dumps a
contextless `Text Result:` blob per tool call, and enables the gitea
SDK's own debug stream (which doesn't merge with our log file). That is
not enough to diagnose a misbehaving tool call in production.

This change introduces:

- A `pkg/middleware.ToolLogging` ToolHandlerMiddleware that logs each
  tool invocation on entry and exit with structured fields: tool name,
  redacted args, request_id, token source, duration_ms, and status. The
  request_id is stashed in the context so downstream layers can
  correlate.
- A `loggingRoundTripper` in `pkg/gitea` that wraps the shared HTTP
  transport and emits one structured line per upstream Gitea API call
  (method, token-stripped URL, status, duration_ms, bytes). This
  replaces — and is far more useful than — `gitea.SetDebugMode()`.
- An effective-config dump at startup (`logEffectiveConfig`) when
  debug+config scope is on, so "did my env var get picked up?" becomes
  answerable from the log file.
- Token source tracking. CLI flag / GITEA_ACCESS_TOKEN /
  GITEA_ACCESS_TOKEN_FILE / per-request Authorization header are now
  distinguished in logs as flag/env/env-file/header.
- A baseline redactor in `pkg/debug` that masks args / query params
  whose keys look like secrets (token, password, secret, api_key,
  authorization, …). This is the placeholder for issue #5's shared
  redactor; once that lands, callers here should defer to it.
- `GITEA_DEBUG_SCOPES` env var to scope debug output to any subset of
  `tools,http,config`. Default (unset) is "everything".

Test coverage was added for the redactor, scope flag, request-id
helpers, preview truncation (multibyte-safe), and the middleware's
context wiring + error propagation.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

pkg/gitea/gitea.go

@@ -18,6 +18,8 @@ var (
 	clientCache     sync.Map // token -> *gitea.Client
 	sharedTransOnce sync.Once
 	sharedTrans     *http.Transport
+	sharedRTOnce    sync.Once
+	sharedRT        http.RoundTripper
 )
 
 func sharedTransport() *http.Transport {
@@ -30,6 +32,22 @@ func sharedTransport() *http.Transport {
 	return sharedTrans
 }
 
+// sharedRoundTripper returns the RoundTripper used by every HTTP client in
+// this package. In debug mode it wraps the underlying transport with a
+// logger that records one entry per upstream Gitea call. Wrapping at the
+// RoundTripper layer means both the SDK-driven path (NewClient) and the
+// hand-rolled REST helpers (DoJSON/DoBytes) get the same instrumentation.
+func sharedRoundTripper() http.RoundTripper {
+	sharedRTOnce.Do(func() {
+		var rt http.RoundTripper = sharedTransport()
+		if flag.Debug {
+			rt = &loggingRoundTripper{base: rt}
+		}
+		sharedRT = rt
+	})
+	return sharedRT
+}
+
 // 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.
@@ -40,16 +58,17 @@ func NewClient(token string) (*gitea.Client, error) {
 	}
 
 	httpClient := &http.Client{
-		Transport:     sharedTransport(),
+		Transport:     sharedRoundTripper(),
 		CheckRedirect: checkRedirect,
 	}
+	// We intentionally don't enable gitea.SetDebugMode() even when
+	// flag.Debug is on: that mode writes to a separate, unstructured stream
+	// that doesn't merge with our zap log file. The loggingRoundTripper
+	// installed above provides equivalent (and structured) visibility.
 	opts := []gitea.ClientOption{
 		gitea.SetToken(token),
 		gitea.SetHTTPClient(httpClient),
 	}
-	if flag.Debug {
-		opts = append(opts, gitea.SetDebugMode())
-	}
 	client, err := gitea.NewClient(flag.Host, opts...)
 	if err != nil {
 		return nil, fmt.Errorf("create gitea client err: %w", err)