Send config_override_yaml instead of budget_usd on run start

[hbrooks]

Why

The Ellipsis backend replaced the per-run budget_usd field with config_override_yaml (a partial agent config merged onto the chosen config and re-validated server-side) in ellipsis-dev/ellipsis#5375. The /v1 run API no longer reads budget_usd, so the CLI must switch over.

What changed

• StartAgentRunRequest: budget_usd?: number → config_override_yaml?: string.
• agent run: the -b, --budget <usd> flag is kept (same ergonomics), but it now emits config_override_yaml set to limits:\n run: <usd> instead of budget_usd. The server merges that onto the config and re-validates, so the budget still resolves from limits.run and is clamped to the $50 platform max as before.
• The unrelated GET /v1/budget usage report (budget_usd on BudgetSummary) is untouched.

Notes

• This was a silent no-op before the fix: the /v1 request model ignores unknown fields, so old CLIs sending budget_usd had no budget applied. After this, --budget works again.
• Only --budget (→ limits.run) is exposed; a general --config-override <yaml> for arbitrary fields can be added later if wanted.
• Should merge after ellipsis-dev/ellipsis#5375 (the server side) is live.

Testing

• npm run typecheck, npm run test (89 tests), and npm run build all pass.

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