Fix agents.json race condition on concurrent starts

Re-read agents.json immediately before writing in startAgent to avoid
one start overwriting another's entry. Also clean stale sockets before
starting new agents.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/overseer.ts

@@ -197,7 +197,7 @@ export async function runOverseer(config: OverseerConfig) {
     for (const name of knownAgents) {
       if (!currentNames.has(name)) {
         log(`Agent "${name}" died, cleaned up.`);
-        bot.say("#agents", `Agent "${name}" has died and been cleaned up.`);
+        bot.say(config.channel, `Agent "${name}" has died and been cleaned up.`);
       }
     }
     knownAgents = currentNames;