fix(k8s): use k8s-native resource quantity units in default manifest

[brettchien]

Summary

Root cause of the Hera pod (namespace openab-studio) sitting in Pending forever with Restarts=0 since this morning's fleet-name-collision incident (see #142).

build_default_k8s_manifest (crates/studio-cp/src/lib.rs) reused the ECS Fargate CPU-unit/MiB default values ("256"/"512") that build_default_manifest (the ECS-path sibling) uses. Those bare numbers are a valid, meaningful convention on the ECS side — manifest.rs's VALID_ECS_CPU whitelist is exactly ["256", "512", "1024", "2048", "4096"], Fargate's CPU units (256 = 0.25 vCPU).

k8s has no such convention. resource_requirements() (crates/oabctl/src/k8s_driver.rs) feeds resources.cpu/.memory straight into a k8s Quantity with no ECS-aware conversion — an unsuffixed "256" parses as 256 whole CPU cores, and "512" as 512 bytes. A pod requesting 256 cores can't be scheduled on any real node, so it sits in Pending indefinitely (Restarts=0 because it never gets past scheduling to even pull the image).

manifest.rs's validate() doesn't catch this: the ECS CPU whitelist check only runs for Runtime::Ecs; for Runtime::Kubernetes it explicitly defers ("K8S: cpu/memory format validated at deploy time by K8S API") — the default builder just never emitted k8s-shaped values for that deferred check to actually validate.

Fix

build_default_k8s_manifest's defaults now use k8s-native quantity strings, sized to the same ~0.25 vCPU / 512Mi as the Fargate default:

cpu: "250m".to_string(),      // was "256" → 256 whole cores
memory: "512Mi".to_string(),  // was "512" → 512 bytes

This is the provision_agent_k8s wizard path (the "New fleet" k8s flow) — exactly what created Hera.

Not fixed here

• The already-live Hera pod is still Pending with the old bad resource request baked into its Deployment spec — this PR only fixes new provisions going forward. Brett will need to redeploy/recreate Hera (or manually patch the Deployment's resources) to pick this up; the pod won't self-heal.

Test plan

• New regression test k8s_manifest_resources_carry_k8s_unit_suffixes asserting the k8s manifest's cpu/memory carry k8s unit suffixes
• cargo test -p studio-cp --lib — 34/34 passing