Phase 2b: Idle disconnect (global default 5s) + Advanced app hold

[itsniper]

Parent: #51

Requirements

• Global idle duration config, default 5 seconds (FR-1.5).
• Idle teardown only when no pending work and no Advanced app hold.
• Advanced: connect(autoReconnect:) sets hold; disconnect() clears hold and intentionally cancels.
• Logging for idle and hold connect/disconnect (FR-9.2).
• DocC: primary vs Advanced.

Acceptance criteria

• Idle fires after default 5s quiet (configurable)
• Hold suppresses idle; disconnect clears hold
• Intentional disconnect does not look like unexpected drop for reconnect policy
• Tests + Advanced docs