feat: add scripts/build-skill.sh to produce claude.ai-upload-ready .skill

Wraps git archive with --prefix=last30days/ so the zip contains a single
top-level skill folder matching SKILL.md's name: frontmatter. Enforces:

- refuses to build with a dirty working tree (prevents shipping untracked changes)
- fails if zip exceeds 200 files (claude.ai's empirical upload cap)
- fails if zip contains more than one SKILL.md (avoids name: confusion)

Output at dist/last30days.skill (gitignored).

scripts/build-skill.sh

@@ -0,0 +1,38 @@
+#!/usr/bin/env bash
+# build-skill.sh - package this repo as a claude.ai-upload-ready .skill file
+# Usage: bash scripts/build-skill.sh  (run from repo root)
+#
+# Produces dist/last30days.skill, a zip with a single top-level `last30days/`
+# directory containing SKILL.md and the scripts/ runtime. See
+# docs/plans/2026-04-14-001-fix-skill-upload-200-file-limit-plan.md.
+set -euo pipefail
+
+REPO_ROOT="$(cd "$(dirname "$0")/.." && pwd)"
+cd "$REPO_ROOT"
+
+if ! git diff --quiet || ! git diff --cached --quiet; then
+  echo "error: working tree is dirty; commit or stash before building" >&2
+  exit 1
+fi
+
+mkdir -p dist
+OUT="dist/last30days.skill"
+git archive --format=zip --prefix=last30days/ --output="$OUT" HEAD
+
+COUNT=$(unzip -l "$OUT" | tail -1 | awk '{print $2}')
+SIZE=$(du -h "$OUT" | cut -f1)
+
+if [ "$COUNT" -gt 200 ]; then
+  echo "error: $COUNT files in zip, claude.ai's cap is 200" >&2
+  echo "       check .gitattributes export-ignore entries" >&2
+  exit 1
+fi
+
+SKILL_MD_COUNT=$(unzip -l "$OUT" | grep -c "SKILL.md" || true)
+if [ "$SKILL_MD_COUNT" -ne 1 ]; then
+  echo "error: expected exactly one SKILL.md, found $SKILL_MD_COUNT" >&2
+  exit 1
+fi
+
+echo "built $OUT ($COUNT files, $SIZE)"
+echo "upload via the claude.ai skill UI"