config: default agent.asset_dir so old configs still serve /assets

Operators who installed vetting before agent.asset_dir existed keep
their config preserved by install.sh on upgrade, which left them
with AssetDir="" — the router silently dropped the /assets/*
mount and the quick-register one-liner hit 404 fetching the agent
binary. Default AssetDir alongside the database file so the same
directory install.sh already creates + drops the agent binary into
is picked up automatically.

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

internal/config/config.go

@@ -3,6 +3,7 @@ package config
 import (
 	"fmt"
 	"os"
+	"path/filepath"
 
 	"gopkg.in/yaml.v3"
 )
@@ -125,5 +126,13 @@ func Load(path string) (*Config, error) {
 	if c.Dispatcher.MaxConcurrentRuns == 0 {
 		c.Dispatcher.MaxConcurrentRuns = 3
 	}
+	// Default the agent asset dir alongside the database file so upgrades
+	// from configs predating the agent.asset_dir field pick up a sensible
+	// location automatically — install.sh creates exactly this directory
+	// and drops vetting-agent-linux-amd64 into it, so /assets/* serves
+	// without the operator having to touch vetting.yaml.
+	if c.Agent.AssetDir == "" {
+		c.Agent.AssetDir = filepath.Join(filepath.Dir(c.Database.Path), "assets")
+	}
 	return &c, nil
 }