feat(health-server): /health checkt /var/cache vrije ruimte

Voeg cacheBytesFree() toe via df -PB1 en retourneer 503 met
{ status: 'unhealthy', reason: 'cache-low' } als < 100 MB vrij.
Bij voldoende ruimte wordt cache_free_bytes toegevoegd aan de
200-response.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

bin/health-server.js

@@ -14,6 +14,7 @@
 const http = require('node:http');
 const fs = require('node:fs/promises');
 const path = require('node:path');
+const { execSync } = require('node:child_process');
 
 const STATE_DIR = process.env.AGENT_STATE_DIR || '/var/run/agent';
 const PORT = Number(process.env.AGENT_HEALTH_PORT || 8080);
@@ -25,6 +26,13 @@ const STATE_FILE = path.join(STATE_DIR, 'state.json');
 const UNHEALTHY_MARKER = path.join(STATE_DIR, 'UNHEALTHY');
 const TOKEN_EXPIRED_MARKER = path.join(STATE_DIR, 'TOKEN_EXPIRED');
 
+const CACHE_LOW_BYTES = 100 * 1024 * 1024; // 100 MB
+
+function cacheBytesFree() {
+    const out = execSync('df -PB1 /var/cache').toString().split('\n')[1];
+    return parseInt(out.split(/\s+/)[3], 10);
+}
+
 async function exists(p) {
     try {
         await fs.access(p);
@@ -59,6 +67,14 @@ async function buildResponse() {
     const heartbeatStale =
         heartbeatAgeS !== null && heartbeatAgeS > STALE_HEARTBEAT_SECONDS;
 
+    const cacheFreeBytes = cacheBytesFree();
+    if (cacheFreeBytes < CACHE_LOW_BYTES) {
+        return {
+            httpStatus: 503,
+            body: { status: 'unhealthy', reason: 'cache-low' },
+        };
+    }
+
     let httpStatus = 200;
     let effectiveStatus = state.status || 'unknown';
 
@@ -87,6 +103,7 @@ async function buildResponse() {
             tokenExpired,
             unhealthy,
         },
+        cache_free_bytes: cacheFreeBytes,
     };
 
     return { httpStatus, body };