Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Adds a spindle CI runner that uses the Fly Machines API to spawn
ephemeral VMs for pipeline execution instead of local Docker.

- flymachines/client.go: Fly Machines REST API client
- flymachines/engine.go: models.Engine implementation using Fly exec API
- flymachines/config.go: SPINDLE_FLY_* env var configuration
- cmd/spindle/main.go: entry point importing spindle server from core
- flymachines/*_test.go: unit tests for client and engine

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>