feat(core): Agent Skills package and skill:// resources

Publish Kubeflow training workflows as an Agent Skill (agentskills.io) and serve them via `skill://` MCP Resources [SEP-2640](https://github.com/modelcontextprotocol/modelcontextprotocol/blob/main/seps/2640-skills-extension.md). Makes Kubeflow discoverable in every major agent harness without MCP tool discovery.

## Context

- Agent Skills is the open standard for portable agent instructions (Claude Code, Codex, Cursor, OpenClaw all read SKILL.md)
- We already have `trainer://guides/*` resources — these are skills in spirit
- Publishing to skills.sh gives immediate discoverability
- SEP-2640 defines `skill://` URI convention for serving skills over MCP

## Deliverables

1. `skills/kubeflow-training/SKILL.md` in repo root (agentskills.io spec compliant)
2. `skill://index.json` MCP resource listing available skills
3. Map existing `trainer://guides/*` -> `skill://kubeflow/*` URIs
4. `mcp.json` in skill package pointing to kubeflow-mcp server

## Acceptance Criteria

- SKILL.md follows spec (YAML frontmatter, progressive disclosure)
- `skill://index.json` served as MCP resource
- Works in Claude Code and Cursor without additional config
- Ready to publish to skills.sh

## References

- [Agent Skills spec](https://agentskills.io/specification)
- Existing: `kubeflow_mcp/trainer/resources/`