Slack channels: opt-in pinned header message instead of setting the description (#508)

Setting the channel description when the bot joins claimed a field members reasonably think of as theirs. The bot now posts a small header message on join (current model plus a settings link) and pins it, instead of calling conversations.setPurpose. On a model change it finds its own pinned header (matched by bot authorship and header shape) and edits it in place rather than posting again. The behavior is opt-in per channel from the web UI's channel settings, and the Slack app manifests gain pins:read/pins:write scopes.

**Deployment notes**

Release-note the Slack app scope update (pins:read/pins:write) + reinstall requirement
Slack app manifest and canonical scope baseline gain pins:read/pins:write — existing
deployments' installed Slack apps lack these scopes until the operator updates the manifest and
reinstalls; the pinned-header feature (opt-in, default off) fails against un-reinstalled apps, so
release notes must say 'update scopes + reinstall before enabling'
Behavior flip even with the toggle off: the bot stops writing the channel description on join, and
the agent display name is dropped from DM topics — visible change for upstream orgs that relied
on the old description behavior
New GET/PUT /v1/channel-header-pin endpoint and a per-channel-scope config flag with change
/ / / p p p p g g g
events — additive config-store key, blue-green safe (old instance ignores the flag)

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