fix(agent): return collected output when exec auto-backgrounds

Previously, when a command exceeded yieldMs (default 5s) and was
auto-backgrounded, exec returned an empty output string. This caused
agents to misinterpret slow commands (like curl) as failed, leading
to infinite retry loops.

Changes:
- Implement three-layer buffer system (pending 30KB + aggregated 200KB + tail 1KB)
- Return collected output snapshot when backgrounding instead of empty string
- Increase default yieldMs from 5s to 10s for better coverage
- Add auto sweeper for terminated process cleanup (30min TTL)
- Register process immediately on spawn to capture all output

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/agent/tools/process.ts

@@ -6,6 +6,7 @@ import {
   PROCESS_REGISTRY,
   registerProcess,
   cleanupTerminatedProcesses,
+  getFullOutput,
 } from "./process-registry.js";
 
 const ProcessSchema = Type.Object({
@@ -138,10 +139,10 @@ export function createProcessTool(defaultCwd?: string): AgentTool<typeof Process
             details: { id, running: false },
           };
         }
-        const output = entry.outputBuffer.join("");
+        const { output, truncated } = getFullOutput(entry);
         const running = entry.exitCode === null;
         return {
-          content: [{ type: "text", text: output || "(no output)" }],
+          content: [{ type: "text", text: (output || "(no output)") + (truncated ? "\n[output truncated]" : "") }],
           details: { id, running, exitCode: entry.exitCode, output },
         };
       }