chore(trainer): clarify namespace binding and CustomTrainer inputs (KEP-936)

[sjiang83]

This PR adds a short "Design Notes (KEP-936 alignment)" block to the first Trainer quickstart example in the README.

It clarifies:

• SDK vs direct API: why an SDK wrapper helps (consistent config/auth surface + Pythonic orchestration).
• Namespace handling: TrainerClient binds namespace via KubernetesBackendConfig(namespace=...) (not per-call).
  • MCP suggestion: expose namespace as an explicit tool parameter and create/cache a per-namespace client internally.
• CustomTrainer inputs: recommend script_code: str over serializing Python callables (with a brief validation/security roadmap).
• Tooling scope: start with orchestration + observability; gate fine-grained tools via persona filtering.

Docs-only; no behavior changes.

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Pull request overview

Adds a “Design notes (KEP-936 alignment)” block to the first Trainer quickstart in the root README to clarify intended SDK vs API usage, namespace binding behavior, and the intended scope of MCP-facing tooling guidance.

Changes:

• Adds a KEP-936 design-notes block before the Trainer quickstart code sample.
• Documents namespace binding as a TrainerClient/KubernetesBackendConfig concern rather than a per-call parameter.
• Introduces guidance on CustomTrainer input format and an MVP tool-surface stance for MCP integrations.