docs: add --disposable flag to automation create examples

[kavinvalli]

Summary

• Adds --disposable flag example to the automation create section in references/REPLICAS.md
• Adds heuristic guidance teaching agents when to use --disposable vs persistent automations
• Disposable automations auto-delete after their first execution, useful for agents creating temporary triggers

Related PRs

• Backend + UI implementation: https://github.com/replicas-group/replicas/pull/1443

Context

Part of the agent-created automations feature in replicas-group/replicas#1443.

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_{Created by Kavin Valli (kavin@replicas.dev) with Replicas
Workspace  ·  Slack Thread}