Migrate classic Slack agents

Turn every agent group wired through the classic instance='slack' adapter into a named slack-<slug> bot without replacing the agent group. This is an operator-guided, resumable data migration. It never creates an agent group and never edits an agent workspace.

Migration is optional. Classic single-bot Slack remains fully supported; this skill first offers the choice, and staying on classic is a valid outcome that also satisfies the update requirement.

Hard invariants:

• Never call create_agent or ncl groups create.
• Never change an agent_groups.id or write under groups/<folder>/.
• Never merge the channels branch; fetch and copy skill-owned files only.
• Never print token values. Show key names and masked presence only.
• Keep classic rows, credentials, and the shared Slack app available for rollback until the operator explicitly approves cutover.