Docs: How to clear previously set attributes for a blueprint

### Doc area

Developer guide (`docs/guides/DEVELOPER_GUIDE.md` or synced site)

### Describe the issue

Non-blocking, follows the existing pattern for every other optional Blueprint field (maxTurns, modelId, systemPromptOverrides all behave identically): because the onUpdate UpdateExpression only SETs the attribute when the prop is present, removing maxBudgetUsd from a Blueprint and redeploying leaves the old max_budget_usd value in the DynamoDB row — the repo keeps its budget cap until someone deletes the attribute out-of-band. Same story on the CLI side: repo-onboard.ts preserves an existing value. Not a defect in this PR (fixing it would mean REMOVE-clause plumbing across all optional fields), but if the docs ever grow a "how do I unset a per-repo budget" question, this is the answer — worth a line in a future docs pass or a tracked issue.

### Affected docs

https://github.com/aws-samples/sample-autonomous-cloud-coding-agents/pull/763#discussion_r3782135860

### Suggested change

_No response_

Lee docs/guides/DEVELOPER_GUIDE.md y la discusión relacionada en PR 763; después inspecciona repo-onboard.ts y la UpdateExpression de onUpdate para confirmar el comportamiento documentado. Se considera completado cuando se haya añadido una respuesta clara sobre cómo desactivar un presupuesto por repositorio, incluidos los pasos necesarios de redistribución o limpieza compatibles con el comportamiento existente del proyecto.

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