Agent +skills source is empty: gke-mcp removed its skills/ directory upstream

The bastion wires the agent's `+skills` capability to a clone of `GoogleCloudPlatform/gke-mcp`, reading skills from that repo's `skills/` directory. That directory no longer exists.

## What happened

`GoogleCloudPlatform/gke-mcp` [#511](https://github.com/GoogleCloudPlatform/gke-mcp/pull/511) ("chore(skills): deprecate and remove skills directory") merged on 2026-08-14 and deleted `skills/` outright. The stated reason is that standalone skill workflows are deprecated, so there is no replacement path to point at. The leftover `.agent/skills` symlink still targets `../skills` and now dangles.

## Why it is quiet

In `scripts/bastion/vm-setup.sh` the clone still succeeds, so the failure never surfaces:

1. `[ -d "${MCP_SKILLS_REPO}/skills" ]` is false
2. `git clone` succeeds, because the repo itself is fine
3. the success message runs `find .../skills -name SKILL.md | wc -l`, which prints `0`

The operator sees `gke-mcp skills ready (0 skills)`. Meanwhile `SKILLS_PATHS` defaults to `$HOME/mcp-skills/skills`, a path that does not exist, so `AGENT_SKILLS_PATHS` points at nothing and the agent runs with no operational skills.

## Impact

Any bastion provisioned after 2026-08-14 runs evals with an empty `+skills`. A bastion with an older clone still has the previous skills on disk and behaves differently, so results are not comparable across machines depending on when each one was set up.

## Options

1. Drop the `+skills` wiring, matching the upstream deprecation
2. Vendor the skills we actually want into this repo and point `SKILLS_PATHS` there
3. Keep the wiring but fail loudly when the resolved skills directory is missing or empty, instead of reporting "ready (0 skills)"

Option 3 is worth doing regardless of which of the first two we choose.

## Context

Found while reviewing #53. That PR removed the stale comment naming four skills that no longer exist, but deliberately did not repoint the skills source, since that is a design change rather than a migration.