Bastion scripts should not default the MCP server and skills to gke-mcp

Per the repo's vendor-neutrality stance, provider-specific tooling should only be selected when the cluster provider actually is that provider. The bastion/matrix scripts currently hardcode GKE tooling as the unconditional default:

- `scripts/bastion/_matrix_lib.sh:65` — `MCP_SERVER_BIN` defaults to `/usr/local/bin/gke-mcp`
- `scripts/bastion/configure-oc.sh:58` — same default, and registers the server under the name `gke-mcp`
- `scripts/bastion/vm-setup.sh` — unconditionally installs gke-mcp and clones its repo as the `+skills` source

Proposed behavior:

- No default MCP server: `+mcp` combos require `MCP_SERVER_BIN` to be set explicitly.
- No default skills source: `+skills` combos load nothing unless `SKILLS_PATHS` is set.
- Install and select `gke-mcp` only when the cluster provider is GKE.

The docs in #123 and #124 already describe the no-default behavior; this issue tracks aligning the scripts.

Start with scripts/bastion/_matrix_lib.sh:65, scripts/bastion/configure-oc.sh:58, and scripts/bastion/vm-setup.sh, then compare their behavior with the no-default guidance in issues #123 and #124. Verify that +mcp requires MCP_SERVER_BIN, +skills requires SKILLS_PATHS, and gke-mcp is installed or selected only for GKE clusters.

Written by the indexing model from the issue text.