Add top-level README and missing productivity bucket README

CLAUDE.md requires every engineering/productivity/misc skill to be
referenced in a top-level README and every bucket to have its own
README, but neither existed for the top level or productivity bucket.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

skills/productivity/README.md

@@ -0,0 +1,16 @@
+# Productivity
+
+Daily non-code workflow tools.
+
+## User-invoked
+
+Reachable only when you type them (`disable-model-invocation: true`).
+
+- **[grill-me](./grill-me/SKILL.md)** — Relentless interview to sharpen a plan or design, for non-code uses.
+- **[handoff](./handoff/SKILL.md)** — Compact the current conversation into a handoff document so another agent can continue the work.
+
+## Model-invoked
+
+Model- or user-reachable (rich trigger phrasing so the model can reach for them).
+
+- **[grilling](./grilling/SKILL.md)** — Interview the user relentlessly about a plan or design until every branch of the decision tree is resolved. The reusable loop behind `grill-me`.