Grafana provider and skills via gcx #96

Evaluate gcx (Apache-2.0) as the Grafana CLI for a Grafana provider and skills.

In favour:

• Release tarballs for linux amd64/arm64.
• Sigstore-signed checksums since v1.6.0.
• A distroless image ghcr.io/grafana/gcx.
• Upstream ships its own skills in claude-plugin/skills.

Concerns:

• The binary is about 104 MB uncompressed. On its own that exceeds kagent's 100 MiB per-root limit, so gcx can only ship in the full root, not the skills-only one.
• gcx reads GRAFANA_SERVER/GRAFANA_TOKEN, but the chart sets GRAFANA_URL/GRAFANA_API_KEY. We need a mapping.
• Verify the Sigstore checksum in the Dockerfile.

Follow-up to #91.