Add handoff skill documentation for agent transition

skills/in-progress/README.md

@@ -2,6 +2,7 @@
 
 Skills that are still being developed. They're not ready to ship — expect rough edges, breaking changes, and abandoned experiments. They're excluded from the plugin and the top-level README until they graduate to a stable bucket.
 
+- **[handoff](./handoff/SKILL.md)** — Compact the current conversation into a handoff document so another agent can continue the work.
 - **[writing-beats](./writing-beats/SKILL.md)** — Shape an article as a journey of beats, choose-your-own-adventure style. Pick a starting beat, write only that beat, then pivot to the next, until the article reaches a natural end.
 - **[writing-fragments](./writing-fragments/SKILL.md)** — Grilling session that mines you for fragments — heterogeneous nuggets of writing — and appends them to a single document as raw material for a future article.
 - **[writing-shape](./writing-shape/SKILL.md)** — Take a markdown file of raw material and shape it into an article paragraph by paragraph, arguing format choices at each step.

skills/in-progress/handoff/SKILL.md

@@ -0,0 +1,12 @@
+---
+name: handoff
+description: Compact the current conversation into a handoff document for another agent to pick up.
+argument-hint: "What will the next session be used for?"
+disable-model-invocation: true
+---
+
+Write a handoff document summarising the current conversation so a fresh agent can continue the work. Save it to a path produced by `mktemp -t handoff-XXXXXX.md`.
+
+Do not duplicate content already captured in other artifacts (PRDs, plans, ADRs, issues, commits, diffs). Reference them by path or URL instead.
+
+If the user passed arguments, treat them as a description of what the next session will focus on and tailor the doc accordingly.