feat(hackmd-deck): add skill

Quick HackMD decks for light talks such as Agoric's engineering show &
tell: a flashback arc (TADA, then how, then challenges), real artifacts
as eye candy, and emoji tags that add up to a retro slide.

In the Portland Pattern Repository form notify-me tried, with an example
skeleton under the h1. The forces come from drafting the Good Fences
deck: the TADA has to be on the talk's own thread; slides show
challenges, they don't pose questions; designed, built in dev, and
deployed must be told apart.

The emoji legend is from the Jun 25 deck; meanings for 🔔 📈 🪄 are
inferred, and 🧱 is new.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

README.md

@@ -43,6 +43,8 @@ Code](https://code.claude.com/docs/en/skills) and
   the link goes; no bare URLs, no "click here".
 - [citation-needed](citation-needed/SKILL.md) — back a challenged claim
   with live links, quoted and dated.
+- [hackmd-deck](hackmd-deck/SKILL.md) — a light talk as a flashback:
+  TADA, then how, then the challenges, tagged for an emoji retro.
 - [ibis-design](ibis-design/SKILL.md) — search a design space:
   issues, positions, and arguments.
 - [obsidian-markdown](obsidian-markdown/SKILL.md) — the workspace is