feat(k8s): deliver config.toml via ConfigMap, drop S3 from the k8s deploy path (studio#138)

[brettchien]

Summary

Brett hit deploy failed ... NoSuchBucket testing a k8s deploy through the New Fleet wizard. Root cause: the k8s path was reusing the exact same S3-backed hooks.pre_seed bundle-carrier the ECS path uses (a deliberate earlier design decision — see the module doc this PR replaces in k8s_driver.rs) — meaning a k8s pod needs AWS credentials just to boot, and a control-plane S3 bucket has to exist at all.

Verified directly against openabdev/openab before writing any of this:

• Every image's Dockerfile.* bakes CMD ["openab", "run", "-c", "/etc/openab/config.toml"] — a plain local path by default.
• load_config_raw_from_source (openab-core/src/config.rs) falls through to a local file read for anything that isn't http(s):///s3://.
• openab's own Helm chart already mounts config.toml via a ConfigMap at /etc/openab and never overrides the command (charts/openab/templates/configmap.yaml + deployment.yaml) — not a new pattern, the upstream project's own existing k8s-native path.

Change

• studio-cp: new provision_config_k8s_configmap (mirrors provision_acp_auth_k8s_secret's exact shape — Patch::Apply, idempotent) server-side-applies config.toml into a k8s ConfigMap, returns a k8s-configmap://<name>#<key> ref. provision_agent_k8s calls this instead of pushing bundle.zip to S3 / injecting a pre_seed hook / resolving a control-plane bucket at all.
• oabctl::secrets: new parse_k8s_configmap_uri, same shape as the existing parse_k8s_secret_uri.
• oabctl::k8s_driver::build_deployment: a k8s-configmap:// configFrom mounts that ConfigMap at /etc/openab and leaves the container command unset (image's own default CMD reads it) — no override, no S3. A legacy s3:///http(s):// configFrom (old stored manifests, and the still-untouched compose-library k8s path) keeps the old override-the-command behavior.
• Dropped the S3-stored-manifest create-vs-redeploy lookup for the wizard's k8s path entirely — every field build_default_k8s_manifest sets is already resent by the wizard on every call, so there was nothing a stored manifest preserved that this doesn't already provide. K8sDriver::apply() already reports Created-vs-Updated from the live Deployment's own existence.

The AWS/ECS path and the older compose-library k8s path (provision_from_library_k8s) are untouched — this only changes the New Fleet wizard's k8s deploy (provision_agent_k8s). deploy_provision_agent's public signature (MCP tool + Tauri bridge) is unchanged.

Test plan

• Added build_deployment unit tests: mounts ConfigMap + skips command override for k8s-configmap://, rejects malformed refs, existing s3:// tests untouched and still pass unchanged (backward-compat path verified by construction, not just claim)
• Added parse_k8s_configmap_uri unit tests mirroring parse_k8s_secret_uri's
• Could not compile locally — this sandbox reliably OOMs on aws-sdk-ec2 regardless of which crate is targeted. Field types (ConfigMap/Volume/VolumeMount/ConfigMapVolumeSource) were verified by reading the pinned k8s-openapi 0.24 source directly, not guessed. CI (build-test) is the real gate — please don't merge until it's green.
• Brett to confirm in a live k8s deploy: no more NoSuchBucket, pod boots with zero AWS credentials

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