Support ticket body carries only the learner's first message #100

Description/Context

SupportBot builds the Zendesk ticket description from the first HumanMessage in the thread, so only the learner's opening line reaches Zendesk. Everything they add while the bot asks follow-up questions is dropped.

That is the wrong half of the conversation to keep. The opening line is typically "my video won't play"; the detail that makes the ticket actionable — which course, which browser, what they already tried — arrives in the turns after it.

Two related cases fall out of the same place:

• A learner whose address is already on the session files on turn 1, with no intake conversation at all, so the ticket is one sentence by construction.
• The intake prompt asks for detail the bot then throws away, which is worse than not asking.

Introduced by #98, where it is listed as a known gap. Inert while AI_ZENDESK_STUB_MODE=True, which is the default.

Plan/Design

Build the description from the whole thread rather than one message. Open questions for whoever picks this up:

• Transcript of every turn, or a summary the bot writes before it files? A transcript is faithful and cheap; a summary reads better to whoever works the ticket.
• The first line is still the natural ticket subject — keep it there.
• Decide what happens on the turn-1 path: either skip the shortcut and always ask at least one follow-up, or accept a thin ticket when the learner has already been identified.