docs(agent): describe the real invite result in the onboarding skill

invite_to_slack_channel never returns a top-level invited: false. It returns invited and refused arrays, and it throws when Slack refuses every address. The skill named a field that does not exist, so the agent could not act on a partial refusal.

Author: ripgrim

apps/agent/agent/skills/customer-onboarding.md

@@ -53,9 +53,15 @@ and decide whether it needs you.
 - **`open_slack_channel` gives back a channel that already exists.** Somebody ran
   this before, or the deal reopened and closed again. Use it. Do not invent a
   second name to get a fresh channel.
-- **`invited: false` with a reason.** Read the reason. "Slack is not connected"
-  and "this workspace does not allow it" both need a person, and neither is
-  fixed by trying again.
+- **`invite_to_slack_channel` gives back a `refused` list.** Some addresses went
+  and some did not. `invited` holds the ones that went; `refused` holds an entry
+  per address, each with the address and a `reason`. Read every reason. "Slack is
+  not connected" and "This workspace doesn't let Comp AI send that invitation"
+  both need a person, and neither is fixed by trying again. Carry on with the
+  people in `invited`, and say plainly who is still outside and why.
+- **`invite_to_slack_channel` fails outright.** Slack refused every address, so
+  the tool errors instead of returning, and the error text is those same reasons.
+  Nobody is in the channel. Do not retry a refusal a person has to clear.
 - **`watching: false`.** This run already watches a different channel, so replies
   in the new one will never reach you. Say so plainly rather than waiting for a
   message that cannot arrive.