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/flag/flag.go

@@ -11,4 +11,30 @@ var (
 	ReadOnly     bool
 	Debug        bool
 	AllowedTools map[string]struct{}
+
+	// TokenSource records where flag.Token was loaded from when the process
+	// started: "flag", "env", "env-file", or "" if no startup token was set.
+	// Per-request tokens from the Authorization header bypass this and are
+	// labelled "header" in the context value instead.
+	TokenSource string
+
+	// DebugScopes restricts what categories of debug output are emitted when
+	// Debug is true. An empty set means "everything" (the default). Known
+	// scopes: "tools", "http", "config". Parsed from GITEA_DEBUG_SCOPES.
+	DebugScopes map[string]struct{}
 )
+
+// DebugScopeEnabled reports whether a given debug scope should be logged.
+// Returns true if Debug is off (so callers don't accidentally activate logs
+// they shouldn't), false. When Debug is on and DebugScopes is empty, all
+// scopes are enabled.
+func DebugScopeEnabled(scope string) bool {
+	if !Debug {
+		return false
+	}
+	if len(DebugScopes) == 0 {
+		return true
+	}
+	_, ok := DebugScopes[scope]
+	return ok
+}