feat(walkthrough): rebuild the walkthrough with Components V2

[phorcys420]

What

Rebuilds the help walkthrough as a single Components V2 message. No more classic embeds, and no more separate lifecycle / resources / question messages.

How

• One message, edited in place through the flow, built by buildWalkthroughMessage:
  • An info container: the data summary (Category / Product / Platform + the logs reminder), the /close + /reopen lifecycle line, and (once the walkthrough is complete) a Docs link button per product resource.
  • A question container and the current selector, shown only while the walkthrough is running.
• State lives in the info container's text, so each step reads it back (readTextDisplays), updates just the answered field (setDataField), and re-renders.
• Documentation buttons now appear only on the last step, and only for products that have resources (Coder yes, code-server no).
• events/walkthrough.ts is now just wiring: ThreadCreate -> doWalkthrough and InteractionCreate -> handleSelection.

Notes

Supersedes #54 (which kept classic embeds). Close #54 in favor of this.

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Opened by Coder Agents on behalf of @phorcys420.