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_

Read docs/guides/DEVELOPER_GUIDE.md and the affected discussion at PR 763, then inspect repo-onboard.ts and the onUpdate UpdateExpression to confirm the documented behavior. Done means adding a clear answer for how to unset a per-repository budget, including any required redeployment or cleanup steps supported by the existing project behavior.

Written by the indexing model from the issue text.