feat(mcp): audit-log every tools/call (OWASP MCP08)

tools/call dispatched with no record, so repo_delete, pr_merge and
file_delete left no trail in gitea-mcp; only auth acceptance was logged.

One slog record per call: tool, caller, token fingerprint, target
(owner/repo/number/branch/path/tag), outcome, duration. Every other
argument is reduced to args_bytes so the audit line never copies file
content or issue bodies.

Closes #64

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/mcp/server.go

@@ -4,6 +4,7 @@ import (
 	"encoding/json"
 	"errors"
 	"net/http"
+	"time"
 
 	"git.d-ma.be/mathias/gitea-mcp/internal/registry"
 )
@@ -90,7 +91,9 @@ func (s *Server) handlePOST(w http.ResponseWriter, r *http.Request) {
 				NewErrorResponse(req.ID, -32602, "invalid params", nil))
 			return
 		}
+		start := time.Now()
 		out, err := s.opts.Registry.Dispatch(r.Context(), p.Name, p.Arguments)
+		logToolCall(r.Context(), p.Name, p.Arguments, start, err)
 		if err != nil {
 			code := -32000
 			if errors.Is(err, registry.ErrToolNotFound) {