feat: wire OpenCode as selectable agent backend via AGENT_BACKEND env var

- Add AGENT_BACKEND config ('container' default, 'opencode' to use OpenCode runtime)
- Wire OpenCodeRuntime into runAgent() with fallback to container runner
- Skip container system check when using OpenCode backend
- Update README and CLAUDE.md with AGENT_BACKEND docs

src/index.ts

@@ -327,7 +327,36 @@ async function runAgent(
   // --- OpenCode backend ---
   if (opencode) {
     try {
-      const response = await opencode.chat(prompt, chatJid);
+      // Read per-group CLAUDE.md as system prompt (same as container backend)
+      const groupDir = path.join(DATA_DIR, '..', 'groups', group.folder);
+      const parts: string[] = [];
+
+      // Load CLAUDE.md
+      const claudeMdPath = path.join(groupDir, 'CLAUDE.md');
+      try {
+        parts.push(fs.readFileSync(claudeMdPath, 'utf-8'));
+        logger.debug({ group: group.name, path: claudeMdPath }, 'Loaded CLAUDE.md as system prompt');
+      } catch {
+        // No CLAUDE.md for this group
+      }
+
+      // Load skills from container/skills/ (same skills the container backend gets)
+      const skillsSrc = path.join(process.cwd(), 'container', 'skills');
+      if (fs.existsSync(skillsSrc)) {
+        for (const skillDir of fs.readdirSync(skillsSrc)) {
+          const srcDir = path.join(skillsSrc, skillDir);
+          if (!fs.statSync(srcDir).isDirectory()) continue;
+          const skillMd = path.join(srcDir, 'SKILL.md');
+          if (fs.existsSync(skillMd)) {
+            parts.push(fs.readFileSync(skillMd, 'utf-8'));
+            logger.debug({ group: group.name, skill: skillDir }, 'Loaded skill');
+          }
+        }
+      }
+
+      const systemPrompt = parts.length > 0 ? parts.join('\n\n---\n\n') : undefined;
+
+      const response = await opencode.chat(prompt, chatJid, systemPrompt);
       if (response.error) {
         logger.error({ group: group.name, error: response.error }, 'OpenCode agent error');
         if (onOutput) {