feat: start selected thread sessions automatically

[paleo]

• The AlignFirst Developer opens a working thread on Slack and Discord only for messages that need one; ordinary conversation is answered in the channel. When a thread is opened, its session starts right after the starter is posted, without a further human message.
• New @paleo/alignfirst-developer-openclaw-plugin (OpenClaw plugin alignfirst-developer): a thread_handoff tool that starts the thread session from a confirmed native starter and claims it once, with SQLite-persisted state, restart recovery, and a thread-handoff list/retire CLI.
• Playbook 0.35.1: selective thread creation, handoff activation and claim before task work, and the supported uses of Slack message send.
• Setup guide 0.33.1: the deployment template installs and enables the plugin, allows its tool, turns off Slack replyToMode and Discord autoThread, and covers the plugin state in backups and updates.
• Test harness: the Slack and Discord mocks follow the native threading contracts, scenarios attribute work to the thread session, two scenarios cover a human reply racing startup and a recoverable delivery failure, and a real-gateway integration suite checks the plugin end to end.
• OpenClaw upgraded to 2026.9.2.
• Maintainer docs and package READMEs updated accordingly.