Reduce idle ACP worker fan-out

[jakehinsy]

Summary

• keep newly created local agents immediate-start, but make future launch-time listening opt-in
• reap manually started lazy ACP harnesses after 15 minutes of inactivity
• preserve existing Auto-start preferences and continuous listeners for explicitly opted-in agents

Root cause

Desktop eagerly launched one lazy buzz-acp harness for every Auto-start agent/community pair. Lazy pooling deferred the model subprocess, but the outer harness and relay socket stayed resident indefinitely, so idle configurations fanned out into dozens of processes.

Behavior notes

• Existing Auto-start settings are unchanged; there is no migration.
• Explicit Auto-start agents retain instant remote-mention behavior.
• Eager/setup-owned harness lifetimes remain caller-owned.

Validation

• just ci on commit 3cd4d1ab74d2bc4d574ef59cb0976f7d259861de
• complete Desktop JavaScript suite passed
• Desktop Rust main crate: 2,980 passed, 18 ignored; integration targets passed
• Mobile: 1,896 passed
• workspace lint, formatting, builds, unit suites, file-size enforcement, web build, and mobile analysis passed

Buzz origin channel: b97e1d62-f607-5ab9-89f8-3ae4363fd9d5

[github-actions]

🔐 Codex Security Review

Status: review required for the current range.

The current range is ed11c8d8bf0a17402be5cf243724f89471530d2f...3cd4d1ab74d2bc4d574ef59cb0976f7d259861de.
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