fix: kill stale sidebar-agent processes before starting new one

Each /connect-chrome starts a new sidebar-agent subprocess with unref()
but never kills the previous one. Old agents accumulate as zombies with
stale auth tokens. When they pick up queue entries, their event relay
fails (401), so the server never receives agent_done and marks the agent
as "hung". The user sees the sidebar freeze.

Fix: pkill any existing sidebar-agent.ts processes before spawning.

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

browse/src/cli.ts

@@ -583,6 +583,14 @@ Refs:           After 'snapshot', use @e1, @e2... as selectors:
           browseBin = process.execPath; // the compiled binary itself
         }
 
+        // Kill any existing sidebar-agent processes before starting a new one.
+        // Old agents have stale auth tokens and will silently fail to relay events,
+        // causing the server to mark the agent as "hung".
+        try {
+          const { spawnSync } = require('child_process');
+          spawnSync('pkill', ['-f', 'sidebar-agent\\.ts'], { stdio: 'ignore', timeout: 3000 });
+        } catch {}
+
         const agentProc = Bun.spawn(['bun', 'run', agentScript], {
           cwd: config.projectDir,
           env: {