Add project vision document

[jmcte]

Summary

• Add an initial VISION.md for github-runner-fleet.
• Capture the project audience, product principles, near-term direction, and non-goals.

Governing Issue

• No issue is linked because this PR was created directly from the operator-injected request to seed first-draft project vision documents for review.
• This is documentation-only planning work and does not close a tracked implementation issue.

Validation

• Not run; documentation-only change.

Bootstrap Governance

• No bootstrap-managed files changed.
• No GitHub policy, runner, or repository settings changed.

Merge Automation

• Not enabled. Please review and merge manually when the draft looks right.

Notes

• Draft v0.1 for review and iteration in this PR.

[pheidon]

Pheidon approval pass: reviewed the new VISION.md. This is documentation-only, matches the runner fleet operational scope, and current required checks are green or intentionally skipped.