Docs: document MentionRequiredInThread channel option (Slack/Discord/Mattermost)

[Aaronontheweb]

Summary

Netclaw adds a new per-channel option MentionRequiredInThread to the Slack, Discord, and Mattermost connectors. The netclaw.dev docs must describe it.

Implemented by netclaw-dev/netclaw#1783. Feature request: netclaw-dev/netclaw#1782.

What the option does

• Default is false. Behavior does not change for existing installs.
• When true, a follow-up reply in a thread with an active session must include an @-mention. Netclaw drops un-mentioned replies.
• A @-mention still continues the existing session as before.
• The option gates both the live-thread path and the daemon-restart rehydration path.

Where to document

• The three channel integration pages (Slack, Discord, Mattermost): add MentionRequiredInThread to the settings list, next to MentionOnly and MentionRequiredInDm.
• The configuration reference: add a row for the option (type bool, default false).

Configuration surface (note for the docs)

• Today you set this option only in netclaw.json, under the channel section. Example key: Slack.MentionRequiredInThread.
• The interactive netclaw init wizard and the netclaw config dashboard do not surface this option yet. Its siblings MentionOnly and MentionRequiredInDm are also JSON-only. Document the JSON path, not a TUI field.

Behavior caveat to verify before you write it

The "read the history on the next mention" behavior depends on actor state:

• If the thread actor is still warm, a later mention does not re-read the un-mentioned messages that arrived in between.
• If the thread actor passivated (about 1 hour idle) or the daemon restarted, the next mention re-reads the gap.

Confirm the final, intended behavior with the maintainers before you document the history-read semantics, because this may change in a follow-up.

[Aaronontheweb]

Update — the feature shipped in a different, cleaner shape than the original #1783 connector-wide bool. Docs should describe the final design:

• Per-channel, not connector-wide. The setting is MentionRequiredInThreadByChannel — a per-channel map (channel ID → bool) on each of the Slack/Discord/Mattermost sections. A channel with no entry defaults to false (today's behavior). There is no connector-wide MentionRequiredInThread (that bool was never deployed and was deleted).
• Backfill on mention. When the rule is on for a channel, an un-mentioned thread reply is ignored; a later mention makes Netclaw re-read everything held since its last reply (it reuses the existing thread-history backfill). So a mention answers with full context — the earlier concern about lost history is resolved.
• Add-time default. When an operator adds a public or team channel, the add-channel step seeds the rule to true (personal channels and DMs stay off). Editable afterward.
• Editable in netclaw config. The per-channel detail leaf (the audience screen) gains a toggle for the rule.

In-repo docs (docs/spec/configuration.md, the three channel integration pages, slack-acl-policy.md, the adding-a-channel runbook) are already updated in the implementation PR; the website should mirror them.