v0.5.0 — cursor coding-agent backend

Adds a third coding-agent backend, cursor, alongside pi and claude_code. It drives Cursor's models through the cursor-agent CLI (cursor-agent login — no API key), proxying Anthropic/OpenAI/Grok/Kimi/Composer under one subscription, behind the same run(...) -> PiResult seam.

Highlights

• coding_agent: cursor with the cursor/ model namespace (cursor/auto, cursor/claude-opus-4-8-thinking-high).
• Own subprocess + tailed NDJSON like pi; tool events re-emitted in pi's shape like agent_cc. agents.execute() is unchanged.
• System prompt carried in the prompt (no --system-prompt flag), composed only on the first send; resumed correction sends carry bare task text.
• anthropic/* still always routes to claude_code, even over an explicit coding_agent: cursor.

Bounded degradations (documented)

• No dollar cost — Cursor bills by subscription; cost is always $0 (tokens exact).
• Inert context-window valve — usage is terminal-only, so a cursor builder relies on the cooperative handoff; execute() notes this at any cursor build phase.

Quality

Validated end-to-end (scout + builder ADWs). Hardened after a high-effort code review (stderr pipe-deadlock fix, system-prompt re-embed fix, dispatch + resume unit tests). just test green: engine pytest 168 · cockpit vitest 72 · roster-mirror + schema parity · typecheck.

Default roster unchanged — sssf.config.yaml ships a commented cursor exemplar only.

PR #40.