feat(kubernetes): prototype driver config passthrough

[elezar]

Summary

Draft POC that demonstrates the RFC #1589 driver_config path end to end: CLI raw JSON, gateway driver-key selection, and Kubernetes driver-local typed config consumption.

This is intentionally not a final Kubernetes config API. The goal is to drive discussion on RFC 0005 / PR #1589 and make the extension boundaries concrete enough to review.

Related Issue

Discussion POC for #1589.

Related to #1492.

Does not close either reference.

Changes

• Adds a pre-POC Docker test cleanup that switches sandbox template fixtures to ..Default::default().
• Adds experimental --driver-config-json CLI plumbing and forwards the selected driver block through the gateway to the compute driver.
• Adds Kubernetes POC handling for a narrow driver-local config shape covering pod scheduling fields and agent container resources.
• Documents the experimental CLI surface and Kubernetes POC shape.
• Notes in the Kubernetes driver that the typed config schema should remain driver-local and may later move to driver-local proto definitions.

Testing

• mise run pre-commit passes
• Unit tests added/updated
• E2E tests added/updated (not run for this POC)

Checklist

• Follows Conventional Commits
• Commits are signed off (DCO)
• Architecture docs updated (not applicable; published/reference docs updated instead)

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