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/context/context.go

@@ -3,5 +3,8 @@ package context
 type contextKey string
 
 const (
-	TokenContextKey = contextKey("token")
+	TokenContextKey       = contextKey("token")
+	TokenSourceContextKey = contextKey("token_source")
+	RequestIDContextKey   = contextKey("request_id")
+	ToolNameContextKey    = contextKey("tool_name")
 )