ci: package skill as installable zip via Gitea Actions

Add a reproducible way to build the Claude-skill package and publish it:

- scripts/build-skill-zip.sh: stages SKILL.md + README + references/ + assets/
  into a zip whose single top-level cycling-training/ folder is the layout
  Claude Desktop / claude.ai import expects. Validates SKILL.md frontmatter.
- .gitea/workflows/release-skill.yml: on tag v* builds the zip and attaches it
  to a Gitea release (via the API using the automatic token); on manual
  dispatch uploads it as a run artifact.
- README: document the packaging/release flow and Claude Desktop zip-import.
- .gitignore: ignore dist/ build output.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01TqrhBhC3GEcKyaw8RTk8G6

scripts/build-skill-zip.sh

@@ -0,0 +1,48 @@
+#!/usr/bin/env bash
+#
+# Package the cycling-training skill into a zip that installs as a Claude skill
+# (Claude Desktop / claude.ai "Skills", or Claude Code).
+#
+# The archive contains a single top-level `cycling-training/` directory holding
+# SKILL.md + supporting docs — the layout Claude expects when importing a skill.
+#
+# Usage:
+#   scripts/build-skill-zip.sh [version]
+# Example:
+#   scripts/build-skill-zip.sh v1.0.0   ->  dist/cycling-training-v1.0.0.zip
+#   scripts/build-skill-zip.sh          ->  dist/cycling-training-dev.zip
+#
+set -euo pipefail
+
+SKILL_NAME="cycling-training"
+ROOT="$(cd "$(dirname "${BASH_SOURCE[0]}")/.." && pwd)"
+VERSION="${1:-dev}"
+OUT_DIR="${ROOT}/dist"
+STAGE="${OUT_DIR}/${SKILL_NAME}"
+ZIP_NAME="${SKILL_NAME}-${VERSION}.zip"
+
+# The files that make up the installable skill (repo/CI cruft is excluded).
+INCLUDE=(SKILL.md README.md references assets)
+
+rm -rf "${OUT_DIR}"
+mkdir -p "${STAGE}"
+
+for item in "${INCLUDE[@]}"; do
+  if [ ! -e "${ROOT}/${item}" ]; then
+    echo "ERROR: expected '${item}' not found at repo root" >&2
+    exit 1
+  fi
+  cp -R "${ROOT}/${item}" "${STAGE}/"
+done
+
+# Sanity checks: a skill is only valid with a SKILL.md that has name + description.
+test -f "${STAGE}/SKILL.md" || { echo "ERROR: SKILL.md missing from package" >&2; exit 1; }
+grep -q "^name:" "${STAGE}/SKILL.md" || { echo "ERROR: SKILL.md has no 'name:' frontmatter" >&2; exit 1; }
+grep -q "^description:" "${STAGE}/SKILL.md" || { echo "ERROR: SKILL.md has no 'description:' frontmatter" >&2; exit 1; }
+
+# Build the zip with the skill folder as the single top-level entry.
+( cd "${OUT_DIR}" && zip -r -q "${ZIP_NAME}" "${SKILL_NAME}" -x '*.DS_Store' )
+
+echo "Built ${OUT_DIR}/${ZIP_NAME}"
+echo "Contents:"
+( cd "${OUT_DIR}" && unzip -l "${ZIP_NAME}" )