Add project vision document

[RayLi-Muye]

Scope

Closes #20.

• Add VISION.md with the project product direction, maintainer experience, public/private safety boundary, near-term priorities, and non-goals.
• Link the vision document from README and docs roadmap.

Acceptance Criteria

• Vision states the target maintainer/user, product promise, safety boundary, and near-term priorities.
• Priorities align with chart contract, local smoke, adapter seams, security examples, GPU scheduling, and chart tests/schema.
• No production cloud resources, credentials, proprietary renderer assets, release, deploy, or package publishing work is introduced.

Validation

• make smoke-local
• PATH=/tmp/codex-renderfarm-tools/bin:$PATH scripts/render-examples.sh
• git diff --check

render-examples.sh performed Helm lint and kubeconform strict validation for default, local, and AWS GPU/SQS rendered manifests.

Docs Impact

• Adds top-level VISION.md.
• README points maintainers to the vision.
• Roadmap points to the vision as the source for prioritization.

Risk / Rollback

Risk is low because this is docs-only. Rollback is reverting this commit to remove VISION.md and the two links.