[P0.17] Enforce or remove the 'Max cost per task (USD)' setting

[frankbria]

Filed by the SaaS launch review. Priority P0.17 — severity high, category unfinished.

Problem

Settings → Agent renders a Max cost per task (USD) control that PUTs to /api/v2/settings and persists config.max_cost_usd, but a full-repo grep finds the field only in config.py, ui/models.py, settings_v2.py and the frontend — no runtime, react_agent, conductor or adapter reads it. A user who sets a $5 cap gets zero cost limiting, and the sibling max_turns field IS wired, making the inert control indistinguishable from a working one. For a paid product a spend cap that silently does nothing is worse than no cap.

Evidence

• web-ui/src/app/settings/page.tsx:319
• codeframe/ui/routers/settings_v2.py:160
• codeframe/core/config.py:167

Acceptance criteria

• Either the execution path enforces max_cost_usd (per-task cost is already tracked) and aborts the run when exceeded, or the field is removed/disabled with an explicit 'not yet enforced' note
• If enforced: a test with a low cap asserts the run terminates and the task is not marked DONE
• No UI control persists a value nothing reads

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