security: stop logging user prompt content on container errors

Container error logs wrote the full ContainerInput (including user
prompt) to disk on every non-zero exit. The structured log stream
also included the first 200 chars of agent output.

- container-runner: only include full input at verbose level; error
  path now logs prompt length and session ID instead
- index: log output length instead of content snippet

Fixes #1150

src/index.ts

@@ -221,7 +221,7 @@ async function processGroupMessages(chatJid: string): Promise<boolean> {
           : JSON.stringify(result.result);
       // Strip <internal>...</internal> blocks — agent uses these for internal reasoning
       const text = raw.replace(/<internal>[\s\S]*?<\/internal>/g, '').trim();
-      logger.info({ group: group.name }, `Agent output: ${raw.slice(0, 200)}`);
+      logger.info({ group: group.name }, `Agent output: ${raw.length} chars`);
       if (text) {
         await channel.sendMessage(chatJid, text);
         outputSentToUser = true;